diff --git a/web/src/invite.ts b/web/src/invite.ts
index 689584ace4..1b5d966c0f 100644
--- a/web/src/invite.ts
+++ b/web/src/invite.ts
@@ -267,10 +267,8 @@ function valid_to(time_valid: number): string {
     return $t({defaultMessage: "Expires on {date} at {time}"}, {date, time});
 }
 
-function set_expires_on_text(): void {
-    const $expires_in = $<HTMLSelectOneElement>("select:not([multiple])#expires_in");
-    if ($expires_in.val() === "custom") {
-        $("#expires_on").hide();
+function set_custom_expires_on_text(): void {
+    if (util.validate_custom_time_input(custom_expiration_time_input)) {
         $("#custom_expires_on").text(
             valid_to(
                 util.get_custom_time_in_minutes(
@@ -279,6 +277,16 @@ function set_expires_on_text(): void {
                 ),
             ),
         );
+    } else {
+        $("#custom_expires_on").text($t({defaultMessage: "Invalid custom time"}));
+    }
+}
+
+function set_expires_on_text(): void {
+    const $expires_in = $<HTMLSelectOneElement>("select:not([multiple])#expires_in");
+    if ($expires_in.val() === "custom") {
+        $("#expires_on").hide();
+        set_custom_expires_on_text();
     } else {
         $("#expires_on").show();
         $("#expires_on").text(valid_to(Number.parseFloat($expires_in.val()!)));
@@ -406,12 +414,14 @@ function open_invite_user_modal(e: JQuery.ClickEvent<Document, undefined>): void
                     .find(".selected")
                     .attr("data-tab-key");
             }
+            const valid_custom_time = util.validate_custom_time_input(custom_expiration_time_input);
             const $button = $("#invite-user-modal .dialog_submit_button");
             $button.prop(
                 "disabled",
-                selected_tab === "invite-email-tab" &&
+                (selected_tab === "invite-email-tab" &&
                     pills.items().length === 0 &&
-                    email_pill.get_current_email(pills) === null,
+                    email_pill.get_current_email(pills) === null) ||
+                    ($expires_in.val() === "custom" && !valid_custom_time),
             );
             if (selected_tab === "invite-email-tab") {
                 $button.text($t({defaultMessage: "Invite"}));
@@ -431,6 +441,7 @@ function open_invite_user_modal(e: JQuery.ClickEvent<Document, undefined>): void
         $expires_in.on("change", () => {
             set_custom_time_inputs_visibility();
             set_expires_on_text();
+            toggle_invite_submit_button();
         });
 
         $("#custom-expiration-time-input").on("keydown", (e) => {
@@ -441,20 +452,14 @@ function open_invite_user_modal(e: JQuery.ClickEvent<Document, undefined>): void
         });
 
         $(".custom-expiration-time").on("change", () => {
-            custom_expiration_time_input = Number.parseFloat(
+            custom_expiration_time_input = util.check_time_input(
                 $<HTMLInputElement>("input#custom-expiration-time-input").val()!,
             );
             custom_expiration_time_unit = $<HTMLSelectOneElement>(
                 "select:not([multiple])#custom-expiration-time-unit",
             ).val()!;
-            $("#custom_expires_on").text(
-                valid_to(
-                    util.get_custom_time_in_minutes(
-                        custom_expiration_time_unit,
-                        custom_expiration_time_input,
-                    ),
-                ),
-            );
+            set_custom_expires_on_text();
+            toggle_invite_submit_button();
         });
 
         $("#invite_check_all_button").on("click", () => {
diff --git a/web/src/util.ts b/web/src/util.ts
index 32e3598054..c1ee1c7f6f 100644
--- a/web/src/util.ts
+++ b/web/src/util.ts
@@ -467,6 +467,13 @@ export function check_time_input(input_value: string, keep_number_as_float = fal
     return Number.parseInt(input_value, 10);
 }
 
+export function validate_custom_time_input(time_input: number): boolean {
+    if (Number.isNaN(time_input) || time_input < 0) {
+        return false;
+    }
+    return true;
+}
+
 // Helper for shorthand for Typescript to get an item from a list with
 // exactly one item.
 export function the<T>(items: T[] | JQuery<T>): T {
diff --git a/web/tests/util.test.js b/web/tests/util.test.js
index f93d8777e5..255a356ac7 100644
--- a/web/tests/util.test.js
+++ b/web/tests/util.test.js
@@ -408,20 +408,24 @@ run_test("check_and_validate_custom_time_input", () => {
     const input_is_nan = "24abc";
     let checked_input = util.check_time_input(input_is_nan);
     assert.equal(checked_input, Number.NaN);
+    assert.equal(util.validate_custom_time_input(checked_input), false);
 
     const input_is_negative = "-24";
     checked_input = util.check_time_input(input_is_negative);
     assert.equal(checked_input, -24);
+    assert.equal(util.validate_custom_time_input(input_is_negative), false);
 
     const input_is_float = "24.5";
     checked_input = util.check_time_input(input_is_float);
     assert.equal(checked_input, 24);
     checked_input = util.check_time_input(input_is_float, true);
     assert.equal(checked_input, 24.5);
+    assert.equal(util.validate_custom_time_input(input_is_float), true);
 
     const input_is_integer = "10";
     checked_input = util.check_time_input(input_is_integer);
     assert.equal(checked_input, 10);
+    assert.equal(util.validate_custom_time_input(input_is_integer), true);
 });
 
 run_test("the", () => {