diff --git a/frontend_tests/node_tests/composebox_typeahead.js b/frontend_tests/node_tests/composebox_typeahead.js
index bf80a0876d..ca34b69381 100644
--- a/frontend_tests/node_tests/composebox_typeahead.js
+++ b/frontend_tests/node_tests/composebox_typeahead.js
@@ -104,9 +104,17 @@ var deactivated_user = {
     user_id: 103,
     full_name: "Deactivated User",
 };
+var lear = {
+    email: 'lear@zulip.com',
+    user_id: 104,
+    full_name: "King Lear",
+};
 
+
+global.people.add_in_realm(hamlet);
 global.people.add_in_realm(othello);
 global.people.add_in_realm(cordelia);
+global.people.add_in_realm(lear);
 global.people.add(deactivated_user);
 
 var hamletcharacters = {
@@ -416,7 +424,7 @@ global.user_groups.add(backend);
     $('#private_message_recipient').typeahead = function (options) {
         // This should match the users added at the beginning of this test file.
         var actual_value = options.source();
-        var expected_value = [othello, cordelia];
+        var expected_value = [hamlet, othello, cordelia, lear];
         assert.deepEqual(actual_value, expected_value);
 
         // Even though the items passed to .highlighter() are the full
@@ -1110,6 +1118,9 @@ global.user_groups.add(backend);
     assert_mentions_matches('cordelia', [cordelia]);
     assert_mentions_matches('cordelia le', [cordelia]);
     assert_mentions_matches('cordelia le ', []);
+    assert_mentions_matches('King ', [hamlet, lear]);
+    assert_mentions_matches('King H', [hamlet]);
+    assert_mentions_matches('King L', [lear]);
     assert_mentions_matches('delia lear', []);
     // Autocomplete user group mentions by group name.
     assert_mentions_matches('hamletchar', [hamletcharacters]);
@@ -1121,7 +1132,8 @@ global.user_groups.add(backend);
     assert_mentions_matches('of hamlet', []);
     // Autocomplete stream by stream name or stream description.
     assert_stream_matches('den', [denmark_stream, sweden_stream]);
-    assert_stream_matches('denmark ', [denmark_stream]);
+    assert_stream_matches('denmark', [denmark_stream]);
+    assert_stream_matches('denmark ', []);
     assert_stream_matches('den ', []);
     assert_stream_matches('cold', [sweden_stream, denmark_stream]);
     assert_stream_matches('the ', [netherland_stream]);
diff --git a/static/js/composebox_typeahead.js b/static/js/composebox_typeahead.js
index 7b0d6ef9df..65fb882647 100644
--- a/static/js/composebox_typeahead.js
+++ b/static/js/composebox_typeahead.js
@@ -67,14 +67,30 @@ function query_matches_source_attrs(query, source, match_attrs, split_char) {
             // query needs to be e.g. 'ab c', not 'cd ef' or 'b cd
             // ef', etc.).
             var queries = query.split(split_char);
+            var sources = source_str.split(split_char);
             var i;
+
             for (i = 0; i < queries.length - 1; i += 1) {
-                if (source_str.split(split_char)[i] !== queries[i]) {
+                // This covers cases where the query is longer than
+                // the target source word.
+                if (sources[i] === undefined) {
+                    return false;
+                }
+                if (sources[i] !== queries[i]) {
                     return false;
                 }
             }
-            return true;
+
+            // This block is effectively a final iteration of the last
+            // loop.  What differs is that for the last word, a
+            // partial match at the beginning of the word is OK.
+            if (sources[i] === undefined) {
+                return false;
+            }
+            return sources[i].indexOf(queries[i]) === 0;
         }
+
+        // For a single token, the match can be anywhere in the string.
         return source_str.indexOf(query) !== -1;
     });
 }