diff --git a/stmhal/main.c b/stmhal/main.c
index 3434f3ea5..ec6d6bcd2 100644
--- a/stmhal/main.c
+++ b/stmhal/main.c
@@ -115,11 +115,13 @@ void nlr_jump_fail(void *val) {
     __fatal_error("");
 }
 
+#ifndef NDEBUG
 void __assert_func(const char *file, int line, const char *func, const char *expr) {
     (void)func;
     printf("Assertion '%s' failed, at file %s:%d\n", expr, file, line);
     __fatal_error("");
 }
+#endif
 
 STATIC mp_obj_t pyb_config_source_dir = MP_OBJ_NULL;
 STATIC mp_obj_t pyb_config_main = MP_OBJ_NULL;