diff --git a/tools/webidl_binder.py b/tools/webidl_binder.py
index 9cdb6a83f57db..d7ce3acfc8c3f 100644
--- a/tools/webidl_binder.py
+++ b/tools/webidl_binder.py
@@ -629,7 +629,8 @@ def make_call_args(i):
     if non_pointer:
       return_prefix += '&'
     if copy:
-      pre += '  static %s temp;\n' % type_to_c(return_type, non_pointing=True)
+      # Avoid sharing this static temp var between threads, which could race.
+      pre += '  static thread_local %s temp;\n' % type_to_c(return_type, non_pointing=True)
       return_prefix += '(temp = '
       return_postfix += ', &temp)'