diff --git a/include/mapnik/util/name_to_int.hpp b/include/mapnik/util/name_to_int.hpp
index 096a1f085..91148d416 100644
--- a/include/mapnik/util/name_to_int.hpp
+++ b/include/mapnik/util/name_to_int.hpp
@@ -22,7 +22,7 @@
 
 namespace mapnik { namespace util {
 
-constexpr unsigned name_to_int(const char *str, int off = 0)
+constexpr unsigned name_to_int(const char *str, unsigned off = 0)
 {
     return !str[off] ? 5381 : (name_to_int(str, off + 1) * 33) ^ static_cast<unsigned>(str[off]);
 }