diff --git a/src/core/os.c b/src/core/os.c
index 21510575..bc7b7bd6 100644
--- a/src/core/os.c
+++ b/src/core/os.c
@@ -1332,7 +1332,7 @@ static Janet os_execute_impl(int32_t argc, Janet *argv, JanetExecuteMode mode) {
                       msgbuf,
                       sizeof(msgbuf),
                       NULL);
-        if (!*msgbuf) snprintf(msgbuf, sizeof(msgbuf), "%" PRIu32, cp_error_code);
+        if (!*msgbuf) snprintf(msgbuf, sizeof(msgbuf), "%" PRIu32, (uint32_t) cp_error_code);
         char *c = msgbuf;
         while (*c) {
             if (*c == '\n' || *c == '\r') {