diff --git a/src/base/group.c b/src/base/group.c
index 31df2d88..3dbdb03e 100644
--- a/src/base/group.c
+++ b/src/base/group.c
@@ -372,8 +372,10 @@ void neu_group_change_test(neu_group_t *group, int64_t timestamp, void *arg,
                            neu_group_change_fn fn)
 {
     if (group->timestamp != timestamp) {
+        pthread_mutex_lock(&group->mtx);
         UT_array *tags = to_array(group->tags);
         fn(arg, group->timestamp, tags, group->interval);
+        pthread_mutex_unlock(&group->mtx);
     }
 }