diff --git a/util/estrtol.c b/util/estrtol.c
index 26c1fde..431b40c 100644
--- a/util/estrtol.c
+++ b/util/estrtol.c
@@ -1,4 +1,5 @@
 /* See LICENSE file for copyright and license details. */
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include "../util.h"
@@ -9,8 +10,9 @@ estrtol(const char *s, int base)
 	char *end;
 	long n;
 	
+	errno = 0;
 	n = strtol(s, &end, base);
-	if(*end != '\0') {
+	if(*end != '\0' || errno != 0) {
 		if(base == 0)
 			eprintf("%s: not an integer\n", s);
 		else