diff --git a/util/cutils.c b/util/cutils.c
index c2250d1ba5..144b25c05a 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -537,16 +537,17 @@ int parse_debug_env(const char *name, int max, int initial)
 {
     char *debug_env = getenv(name);
     char *inv = NULL;
-    int debug;
+    long debug;
 
     if (!debug_env) {
         return initial;
     }
+    errno = 0;
     debug = strtol(debug_env, &inv, 10);
     if (inv == debug_env) {
         return initial;
     }
-    if (debug < 0 || debug > max) {
+    if (debug < 0 || debug > max || errno != 0) {
         fprintf(stderr, "warning: %s not in [0, %d]", name, max);
         return initial;
     }