diff --git a/libs/base/protos/message.proto b/libs/base/protos/message.proto
index 668dfff06..991e8a31e 100644
--- a/libs/base/protos/message.proto
+++ b/libs/base/protos/message.proto
@@ -310,7 +310,9 @@ message KeyEvent {
 }
 
 message CursorData {
-  uint64 id = 1;
+  // A string in JS: a Windows peer before 1.5.0 sends its cursor handle, sign-extended above
+  // 2^53 when its top bit is set, which a JS number cannot hold.
+  uint64 id = 1 [jstype = JS_STRING];
   sint32 hotx = 2;
   sint32 hoty = 3;
   int32 width = 4;
@@ -1008,7 +1010,7 @@ message Message {
     AudioFrame audio_frame = 11;
     CursorData cursor_data = 12;
     CursorPosition cursor_position = 13;
-    uint64 cursor_id = 14;
+    uint64 cursor_id = 14 [jstype = JS_STRING]; // as CursorData.id
     KeyEvent key_event = 15;
     Clipboard clipboard = 16;
     FileAction file_action = 17;