diff --git a/NmapOps.cc b/NmapOps.cc
index 708c300cb..1f881ef5d 100644
--- a/NmapOps.cc
+++ b/NmapOps.cc
@@ -190,7 +190,7 @@ char *filename_to_url(const char *filename) {
 #endif
 
   /* Percent-encode any troublesome characters. */
-  unsigned int i = 0;
+  std::string::size_type i = 0;
   /* See RFC 3986, section 3.3 "Path" for allowed characters. */
   while ((i = url.find_first_of("?#[]%", i)) != std::string::npos) {
     Snprintf(percent_buffer, sizeof(percent_buffer), "%%%02X", url[i]);