diff --git a/include/jemalloc/internal/mutex.h b/include/jemalloc/internal/mutex.h
index ceb07b85..db2bdf37 100644
--- a/include/jemalloc/internal/mutex.h
+++ b/include/jemalloc/internal/mutex.h
@@ -250,6 +250,9 @@ malloc_mutex_unlock(tsdn_t *tsdn, malloc_mutex_t *mutex) {
 static inline void
 malloc_mutex_assert_owner(tsdn_t *tsdn, malloc_mutex_t *mutex) {
 	witness_assert_owner(tsdn_witness_tsdp_get(tsdn), &mutex->witness);
+	if (isthreaded) {
+		assert(malloc_mutex_is_locked(mutex));
+	}
 }
 
 static inline void