diff --git a/web/src/filter.ts b/web/src/filter.ts
index 77fbf9308f..3c7d62746d 100644
--- a/web/src/filter.ts
+++ b/web/src/filter.ts
@@ -1433,19 +1433,18 @@ export class Filter {
         if (term_types.length === 1 && _.isEqual(term_types, ["sender"])) {
             const email = this.operands("sender")[0]!;
             const user = people.get_by_email(email);
-            let sender = email;
-            if (user) {
-                if (people.is_my_user_id(user.user_id)) {
-                    return $t({defaultMessage: "Messages sent by you"});
-                }
-
-                if (people.should_add_guest_user_indicator(user.user_id)) {
-                    sender = $t({defaultMessage: "{name} (guest)"}, {name: user.full_name});
-                } else {
-                    sender = user.full_name;
-                }
+            if (user === undefined) {
+                return $t({defaultMessage: "Messages sent by unknown user"});
+            }
+            if (people.is_my_user_id(user.user_id)) {
+                return $t({defaultMessage: "Messages sent by you"});
+            }
+            let sender: string;
+            if (people.should_add_guest_user_indicator(user.user_id)) {
+                sender = $t({defaultMessage: "{name} (guest)"}, {name: user.full_name});
+            } else {
+                sender = user.full_name;
             }
-
             return $t(
                 {defaultMessage: "Messages sent by {sender}"},
                 {
diff --git a/web/tests/filter.test.cjs b/web/tests/filter.test.cjs
index 3d435075da..c9745e95a0 100644
--- a/web/tests/filter.test.cjs
+++ b/web/tests/filter.test.cjs
@@ -2337,6 +2337,7 @@ test("navbar_helpers", ({override}) => {
 
     const sender = [{operator: "sender", operand: joe.email}];
     const guest_sender = [{operator: "sender", operand: alice.email}];
+    const invalid_sender = [{operator: "sender", operand: "sally@doesnotexist.co"}];
     const in_home = [{operator: "in", operand: "home"}];
     const in_all = [{operator: "in", operand: "all"}];
     const is_starred = [{operator: "is", operand: "starred"}];
@@ -2434,6 +2435,13 @@ test("navbar_helpers", ({override}) => {
             title: "translated: Messages sent by translated: alice (guest)",
             redirect_url_with_search: "/#narrow/sender/" + alice.user_id + "-alice",
         },
+        {
+            terms: invalid_sender,
+            is_common_narrow: true,
+            icon: undefined,
+            title: "translated: Messages sent by unknown user",
+            redirect_url_with_search: "/#narrow/sender/undefined",
+        },
         {
             terms: is_starred,
             is_common_narrow: true,