diff --git a/src/or/test.c b/src/or/test.c
index 21916c8a0e..f5e9d51230 100644
--- a/src/or/test.c
+++ b/src/or/test.c
@@ -1229,7 +1229,9 @@ _thread_test_func(void* _s)
     strmap_set(_thread_test_strmap, "last to run", cp);
     tor_mutex_release(_thread_test_mutex);
   }
+  tor_mutex_acquire(_thread_test_mutex);
   strmap_set(_thread_test_strmap, s, tor_strdup(buf));
+  tor_mutex_release(_thread_test_mutex);
 
   tor_mutex_release(m);