diff --git a/test/cfoa/visit_tests.cpp b/test/cfoa/visit_tests.cpp
index 6660fe7d..d604bffa 100644
--- a/test/cfoa/visit_tests.cpp
+++ b/test/cfoa/visit_tests.cpp
@@ -29,7 +29,8 @@ namespace {
   template<typename T>
   raii const & get_non_present_key(T const & x)
   {
-    return non_present_keys[get_key(x).x_ % non_present_keys.size()];
+    return non_present_keys[
+      (std::size_t)get_key(x).x_ % non_present_keys.size()];
   }
 
   struct lvalue_visitor_type