diff --git a/include/internal/catch_test_case_registry_impl.hpp b/include/internal/catch_test_case_registry_impl.hpp
index 316c6c2c..c32cc9f1 100644
--- a/include/internal/catch_test_case_registry_impl.hpp
+++ b/include/internal/catch_test_case_registry_impl.hpp
@@ -22,7 +22,7 @@
 namespace Catch {
 
     struct RandomNumberGenerator {
-        typedef std::ptrdiff_t result_type;
+        typedef std::make_unsigned<std::ptrdiff_t>::type result_type;
 
         result_type operator()( result_type n ) const { return std::rand() % n; }