diff --git a/src/support/filetools.C b/src/support/filetools.C
index 2b59efe323..6b09c1dcdb 100644
--- a/src/support/filetools.C
+++ b/src/support/filetools.C
@@ -213,7 +213,7 @@ vector<string> const DirList(string const & dir, string const & ext)
 	// This is a non-error checking C/system implementation
 	string extension(ext);
 	if (!extension.empty() && extension[0] != '.')
-		extension.insert(0, 1, ".");
+		extension.insert(0u, 1, '.');
 	vector<string> dirlist;
 	DIR * dirp = ::opendir(dir.c_str());
 	if (!dirp) {