diff --git a/src/timespec.c b/src/timespec.c
index 953b15a..a4b2ea6 100644
--- a/src/timespec.c
+++ b/src/timespec.c
@@ -240,7 +240,7 @@ bool timespec_get_spec(struct mgos_timespec *ts, char *ret, int retlen) {
 
     snprintf(spec_str, sizeof(spec_str) - 1, "%02d:%02d:%02d-%02d:%02d:%02d", t_spec->start_h, t_spec->start_m, t_spec->start_s,
              t_spec->stop_h, t_spec->stop_m, t_spec->stop_s);
-    if (strlen(spec_str) + strlen(ret) > retlen - 1) {
+    if ((int) (strlen(spec_str) + strlen(ret)) > retlen - 1) {
       return false;
     }
     if (strlen(ret) > 0) {