diff --git a/web/src/buddy_data.js b/web/src/buddy_data.js
index d1f071432d..12a9247b3e 100644
--- a/web/src/buddy_data.js
+++ b/web/src/buddy_data.js
@@ -75,8 +75,8 @@ export function compare_function(a, b) {
     }
 
     // Sort equivalent direct message names alphabetically
-    const person_a = people.get_by_user_id(a);
-    const person_b = people.get_by_user_id(b);
+    const person_a = people.maybe_get_user_by_id(a);
+    const person_b = people.maybe_get_user_by_id(b);
 
     const full_name_a = person_a ? person_a.full_name : "";
     const full_name_b = person_b ? person_b.full_name : "";
@@ -253,7 +253,7 @@ function filter_user_ids(user_filter_text, user_ids) {
     // This first filter is for whether the user is eligible to be
     // displayed in the right sidebar at all.
     user_ids = user_ids.filter((user_id) => {
-        const person = people.get_by_user_id(user_id);
+        const person = people.maybe_get_user_by_id(user_id);
 
         if (!person) {
             blueslip.warn("Got user_id in presence but not people: " + user_id);
diff --git a/web/src/compose_validate.js b/web/src/compose_validate.js
index e779733693..3aa6e5a615 100644
--- a/web/src/compose_validate.js
+++ b/web/src/compose_validate.js
@@ -39,7 +39,7 @@ export function needs_subscribe_warning(user_id, stream_id) {
     //  We expect the caller to already have verified that we're
     //  sending to a valid stream and trying to mention the user.
 
-    const user = people.get_by_user_id(user_id);
+    const user = people.maybe_get_user_by_id(user_id);
 
     if (!user) {
         return false;
diff --git a/web/src/message_helper.js b/web/src/message_helper.js
index bdeffa946f..4ac7cbc91c 100644
--- a/web/src/message_helper.js
+++ b/web/src/message_helper.js
@@ -29,7 +29,7 @@ export function process_new_message(message) {
     people.extract_people_from_message(message);
     people.maybe_incr_recipient_count(message);
 
-    const sender = people.get_by_user_id(message.sender_id);
+    const sender = people.maybe_get_user_by_id(message.sender_id);
     if (sender) {
         message.sender_full_name = sender.full_name;
         message.sender_email = sender.email;
diff --git a/web/src/message_store.js b/web/src/message_store.js
index 2961b17f08..eaeeef6e69 100644
--- a/web/src/message_store.js
+++ b/web/src/message_store.js
@@ -39,7 +39,7 @@ export function get_pm_emails(message) {
     const user_ids = people.pm_with_user_ids(message);
     const emails = user_ids
         .map((user_id) => {
-            const person = people.get_by_user_id(user_id);
+            const person = people.maybe_get_user_by_id(user_id);
             if (!person) {
                 blueslip.error("Unknown user id", {user_id});
                 return "?";
diff --git a/web/src/peer_data.js b/web/src/peer_data.js
index ab85c2e4ca..bf37e07a23 100644
--- a/web/src/peer_data.js
+++ b/web/src/peer_data.js
@@ -98,7 +98,7 @@ export function add_subscriber(stream_id, user_id) {
     // If stream_id/user_id are unknown to us, we will
     // still track it, but we will warn.
     const subscribers = get_user_set(stream_id);
-    const person = people.get_by_user_id(user_id);
+    const person = people.maybe_get_user_by_id(user_id);
     if (person === undefined) {
         blueslip.warn("We tried to add invalid subscriber: " + user_id);
     }
diff --git a/web/src/people.js b/web/src/people.js
index 19534aed68..bc3220c7d3 100644
--- a/web/src/people.js
+++ b/web/src/people.js
@@ -1,5 +1,6 @@
 import md5 from "blueimp-md5";
 import {format, utcToZonedTime} from "date-fns-tz";
+import assert from "minimalistic-assert";
 
 import * as typeahead from "../shared/src/typeahead";
 
@@ -58,9 +59,17 @@ export function get_users_from_ids(user_ids) {
     return user_ids.map((user_id) => get_by_user_id(user_id));
 }
 
-export function get_by_user_id(user_id, ignore_missing) {
-    if (!people_by_user_id_dict.has(user_id) && !ignore_missing) {
-        blueslip.error("Unknown user_id in get_by_user_id", {user_id});
+// Use this function only when you are sure that user_id is valid.
+export function get_by_user_id(user_id) {
+    const person = people_by_user_id_dict.get(user_id);
+    assert(person, `Unknown user_id in get_by_user_id: ${user_id}`);
+    return person;
+}
+
+// This is type unsafe version of get_by_user_id for the callers that expects undefined values.
+export function maybe_get_user_by_id(user_id) {
+    if (!people_by_user_id_dict.has(user_id)) {
+        blueslip.error("Unknown user_id in maybe_get_user_by_id", {user_id});
         return undefined;
     }
     return people_by_user_id_dict.get(user_id);
@@ -347,7 +356,7 @@ export function get_full_names_for_poll_option(user_ids) {
 }
 
 export function get_display_full_name(user_id) {
-    const person = get_by_user_id(user_id);
+    const person = maybe_get_user_by_id(user_id);
     if (!person) {
         blueslip.error("Unknown user id", {user_id});
         return "?";
@@ -528,7 +537,7 @@ export function pm_with_url(message) {
     if (user_ids.length > 1) {
         suffix = "group";
     } else {
-        const person = get_by_user_id(user_ids[0]);
+        const person = maybe_get_user_by_id(user_ids[0]);
         if (person && person.full_name) {
             suffix = person.full_name.replaceAll(/[ "%/<>`\p{C}]+/gu, "-");
         } else {
@@ -772,7 +781,7 @@ export function small_avatar_url(message) {
         // We should always have message.sender_id, except for in the
         // tutorial, where it's ok to fall back to the URL in the fake
         // messages.
-        person = get_by_user_id(message.sender_id);
+        person = maybe_get_user_by_id(message.sender_id);
     }
 
     // The first time we encounter a sender in a message, we may
diff --git a/web/src/popovers.js b/web/src/popovers.js
index 8313c1e9c7..6f9144767e 100644
--- a/web/src/popovers.js
+++ b/web/src/popovers.js
@@ -450,7 +450,7 @@ function get_user_info_popover_manage_menu_items() {
 
 function fetch_group_members(member_ids) {
     return member_ids
-        .map((m) => people.get_by_user_id(m))
+        .map((m) => people.maybe_get_user_by_id(m))
         .filter((m) => m !== undefined)
         .map((p) => ({
             ...p,
diff --git a/web/src/presence.js b/web/src/presence.js
index 58c95fa638..6b240e69a2 100644
--- a/web/src/presence.js
+++ b/web/src/presence.js
@@ -175,7 +175,7 @@ export function set_info(presences, server_timestamp) {
         // reload_state.is_in_progress, because races where presence
         // returns data on users not yet received via the server_events
         // system are common in both situations.
-        const person = people.get_by_user_id(user_id, true);
+        const person = people.maybe_get_user_by_id(user_id);
         if (person === undefined) {
             if (!(watchdog.suspects_user_is_offline() || reload_state.is_in_progress())) {
                 // If we're online, and we get a user who we don't
diff --git a/web/src/rendered_markdown.js b/web/src/rendered_markdown.js
index df05aad7d7..764ebe3df4 100644
--- a/web/src/rendered_markdown.js
+++ b/web/src/rendered_markdown.js
@@ -107,7 +107,7 @@ export const update_elements = ($content) => {
             // mention text to show the user's current name,
             // assuming that you're not searching for text
             // inside the highlight.
-            const person = people.get_by_user_id(user_id, true);
+            const person = people.maybe_get_user_by_id(user_id);
             if (person !== undefined) {
                 // Note that person might be undefined in some
                 // unpleasant corner cases involving data import.
diff --git a/web/src/settings_bots.js b/web/src/settings_bots.js
index cb07a7fe4a..cfb4b4d03f 100644
--- a/web/src/settings_bots.js
+++ b/web/src/settings_bots.js
@@ -340,7 +340,7 @@ export function confirm_bot_deactivation(bot_id, handle_confirm, loading_spinner
 }
 
 export function show_edit_bot_info_modal(user_id, from_user_info_popover) {
-    const bot = people.get_by_user_id(user_id);
+    const bot = people.maybe_get_user_by_id(user_id);
     const owner_id = bot_data.get(user_id).owner_id;
     const owner_full_name = people.get_full_name(owner_id);
 
diff --git a/web/src/settings_emoji.js b/web/src/settings_emoji.js
index 97acc96ca5..fae0bd5e4e 100644
--- a/web/src/settings_emoji.js
+++ b/web/src/settings_emoji.js
@@ -102,7 +102,7 @@ export function populate_emoji() {
     for (const emoji of Object.values(emoji_data)) {
         // Add people.js data for the user here.
         if (emoji.author_id !== null) {
-            emoji.author = people.get_by_user_id(emoji.author_id);
+            emoji.author = people.maybe_get_user_by_id(emoji.author_id);
         } else {
             emoji.author = null;
         }
diff --git a/web/src/settings_users.js b/web/src/settings_users.js
index 93408538d3..170bebd769 100644
--- a/web/src/settings_users.js
+++ b/web/src/settings_users.js
@@ -191,7 +191,7 @@ function bot_owner_full_name(owner_id) {
         return undefined;
     }
 
-    const bot_owner = people.get_by_user_id(owner_id);
+    const bot_owner = people.maybe_get_user_by_id(owner_id);
     if (!bot_owner) {
         return undefined;
     }
@@ -200,7 +200,7 @@ function bot_owner_full_name(owner_id) {
 }
 
 function bot_info(bot_user_id) {
-    const bot_user = people.get_by_user_id(bot_user_id);
+    const bot_user = people.maybe_get_user_by_id(bot_user_id);
 
     if (!bot_user) {
         return undefined;
@@ -616,7 +616,7 @@ function handle_reactivation($tbody) {
 }
 
 export function show_edit_user_info_modal(user_id, from_user_info_popover) {
-    const person = people.get_by_user_id(user_id);
+    const person = people.maybe_get_user_by_id(user_id);
 
     if (!person) {
         return;
diff --git a/web/src/stream_create_subscribers_data.js b/web/src/stream_create_subscribers_data.js
index 33648755f4..53e4a7f03e 100644
--- a/web/src/stream_create_subscribers_data.js
+++ b/web/src/stream_create_subscribers_data.js
@@ -40,7 +40,7 @@ export function must_be_subscribed(user_id) {
 export function add_user_ids(user_ids) {
     for (const user_id of user_ids) {
         if (!user_id_set.has(user_id)) {
-            const user = people.get_by_user_id(user_id);
+            const user = people.maybe_get_user_by_id(user_id);
             if (user) {
                 user_id_set.add(user_id);
             }
diff --git a/web/src/user_events.js b/web/src/user_events.js
index ca9af2a815..caea60b91b 100644
--- a/web/src/user_events.js
+++ b/web/src/user_events.js
@@ -22,7 +22,7 @@ import * as settings_streams from "./settings_streams";
 import * as settings_users from "./settings_users";
 
 export const update_person = function update(person) {
-    const person_obj = people.get_by_user_id(person.user_id);
+    const person_obj = people.maybe_get_user_by_id(person.user_id);
 
     if (!person_obj) {
         blueslip.error("Got update_person event for unexpected user", {user_id: person.user_id});
diff --git a/web/src/user_group_create_members_data.js b/web/src/user_group_create_members_data.js
index 66e34973d4..c24a871548 100644
--- a/web/src/user_group_create_members_data.js
+++ b/web/src/user_group_create_members_data.js
@@ -36,7 +36,7 @@ export function get_potential_members() {
 export function add_user_ids(user_ids) {
     for (const user_id of user_ids) {
         if (!user_id_set.has(user_id)) {
-            const user = people.get_by_user_id(user_id);
+            const user = people.maybe_get_user_by_id(user_id);
             if (user) {
                 user_id_set.add(user_id);
             }
diff --git a/web/tests/buddy_data.test.js b/web/tests/buddy_data.test.js
index f85fb47be6..e9bec489f5 100644
--- a/web/tests/buddy_data.test.js
+++ b/web/tests/buddy_data.test.js
@@ -549,7 +549,7 @@ test("get_items_for_users", () => {
 
 test("error handling", () => {
     presence.presence_info.set(42, {status: "active"});
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     blueslip.expect("warn", "Got user_id in presence but not people: 42");
     buddy_data.get_filtered_and_sorted_user_ids();
 });
diff --git a/web/tests/compose_validate.test.js b/web/tests/compose_validate.test.js
index cbc6cba280..15467b7140 100644
--- a/web/tests/compose_validate.test.js
+++ b/web/tests/compose_validate.test.js
@@ -616,7 +616,7 @@ test_ui("needs_subscribe_warning", () => {
     stream_data.add_sub(sub);
     peer_data.set_subscribers(sub.stream_id, [bob.user_id, me.user_id]);
 
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     // Test with an invalid user id.
     assert.equal(compose_validate.needs_subscribe_warning(invalid_user_id, sub.stream_id), false);
 
diff --git a/web/tests/message_store.test.js b/web/tests/message_store.test.js
index f3b7771d37..32238cb2bf 100644
--- a/web/tests/message_store.test.js
+++ b/web/tests/message_store.test.js
@@ -212,7 +212,7 @@ test("errors", ({disallow_rewire}) => {
         display_recipient: [{id: 92714}],
     };
 
-    blueslip.expect("error", "Unknown user_id in get_by_user_id", 2);
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id", 2);
     blueslip.expect("error", "Unknown user id", 2); // From person.js
 
     // Expect each to throw two blueslip errors
diff --git a/web/tests/peer_data.test.js b/web/tests/peer_data.test.js
index 894226ca18..a92704de1b 100644
--- a/web/tests/peer_data.test.js
+++ b/web/tests/peer_data.test.js
@@ -204,7 +204,7 @@ test("subscribers", () => {
     blueslip.reset();
 
     // Verify that we don't crash for a bad user id.
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     blueslip.expect("warn", "We tried to add invalid subscriber: 88888");
     peer_data.add_subscriber(stream_id, 88888);
     blueslip.reset();
diff --git a/web/tests/people.test.js b/web/tests/people.test.js
index 50a849df8a..656a259f82 100644
--- a/web/tests/people.test.js
+++ b/web/tests/people.test.js
@@ -795,7 +795,7 @@ test_people("message_methods", () => {
         "https://secure.gravatar.com/avatar/6dbdd7946b58d8b11351fcb27e5cdd55?d=identicon&s=50",
     );
 
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     message = {
         avatar_url: undefined,
         sender_email: "foo@example.com",
diff --git a/web/tests/people_errors.test.js b/web/tests/people_errors.test.js
index 03e76ec063..597528cab2 100644
--- a/web/tests/people_errors.test.js
+++ b/web/tests/people_errors.test.js
@@ -101,7 +101,7 @@ run_test("blueslip", () => {
     const reply_to = people.pm_reply_to(message);
     assert.ok(reply_to.includes("?"));
 
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     blueslip.expect("error", "Unknown people in message");
     const url = people.pm_with_url({type: "private", display_recipient: [{id: 42}]});
     assert.equal(url.indexOf("unk"), url.length - 3);
diff --git a/web/tests/presence.test.js b/web/tests/presence.test.js
index 8fb2c5362f..89745a0e18 100644
--- a/web/tests/presence.test.js
+++ b/web/tests/presence.test.js
@@ -95,6 +95,7 @@ test("unknown user", ({override}) => {
     const presences = {};
     presences[unknown_user_id.toString()] = "does-not-matter";
 
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id", 3);
     blueslip.expect("error", "Unknown user ID in presence data");
     presence.set_info(presences, now);
 
diff --git a/web/tests/user_events.test.js b/web/tests/user_events.test.js
index 3172bb5880..33ce685b8e 100644
--- a/web/tests/user_events.test.js
+++ b/web/tests/user_events.test.js
@@ -217,7 +217,7 @@ run_test("updates", () => {
     assert.ok(person.timezone);
 
     blueslip.expect("error", "Got update_person event for unexpected user");
-    blueslip.expect("error", "Unknown user_id in get_by_user_id");
+    blueslip.expect("error", "Unknown user_id in maybe_get_user_by_id");
     assert.ok(!user_events.update_person({user_id: 29, full_name: "Sir Isaac Newton"}));
 
     me.profile_data = {};