diff --git a/web/src/narrow_state.js b/web/src/narrow_state.js
index 5daec76896..a1e8bc7a1d 100644
--- a/web/src/narrow_state.js
+++ b/web/src/narrow_state.js
@@ -88,7 +88,13 @@ export function set_compose_defaults() {
     // if they are uniquely specified in the narrow view.
 
     if (single.has("stream")) {
-        opts.stream = stream_data.get_name(single.get("stream"));
+        // The raw stream name from collect_single may be an arbitrary
+        // unvalidated string from the URL fragment and thus not be valid.
+        // So we look up the resolved stream and return that if appropriate.
+        const sub = stream_sub();
+        if (sub !== undefined) {
+            opts.stream = sub.name;
+        }
     }
 
     if (single.has("topic")) {
diff --git a/web/tests/narrow_state.test.js b/web/tests/narrow_state.test.js
index e78e37ef89..12b62920bb 100644
--- a/web/tests/narrow_state.test.js
+++ b/web/tests/narrow_state.test.js
@@ -174,7 +174,14 @@ test("set_compose_defaults", () => {
         ["topic", "Bar"],
     ]);
 
-    const stream_and_topic = narrow_state.set_compose_defaults();
+    // First try with a stream that doesn't exist.
+    let stream_and_topic = narrow_state.set_compose_defaults();
+    assert.equal(stream_and_topic.stream, undefined);
+    assert.equal(stream_and_topic.topic, "Bar");
+
+    const test_stream = {name: "Foo", stream_id: 72};
+    stream_data.add_sub(test_stream);
+    stream_and_topic = narrow_state.set_compose_defaults();
     assert.equal(stream_and_topic.stream, "Foo");
     assert.equal(stream_and_topic.topic, "Bar");