diff --git a/py/modstruct.c b/py/modstruct.c
index 95a282e2ec1746a8bf774d97bf017809a7d4899c..19cf9cfd6c05064486ee0f30f9f27794359c6f70 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -85,7 +85,7 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
     char fmt_type = get_fmt_type(&fmt);
     machine_uint_t size;
     for (size = 0; *fmt; fmt++) {
-        uint align;
+        uint align = 1;
         machine_uint_t cnt = 1;
         if (unichar_isdigit(*fmt)) {
             cnt = get_fmt_num(&fmt);