diff --git a/src/iscidraddr.c b/src/iscidraddr.c
index ccd01ae..3d889bd 100644
--- a/src/iscidraddr.c
+++ b/src/iscidraddr.c
@@ -35,6 +35,10 @@ in_range(char buf[4], int min, int max)
   char *err;
   long r;
   errno = 0;
-  r = strtol(buf, &err, 10);
-  return *err == '\0' && errno == 0 && (r >= min && r <= max);
+  if (strnlen(buf, 1) == 0) {
+    return 0;
+  } else {
+    r = strtol(buf, &err, 10);
+    return *err == '\0' && errno == 0 && (r >= min && r <= max);
+  }
 }