diff --git a/lib/virtus/support/type_lookup.rb b/lib/virtus/support/type_lookup.rb
index a6ce2e39..ecb88690 100644
--- a/lib/virtus/support/type_lookup.rb
+++ b/lib/virtus/support/type_lookup.rb
@@ -32,7 +32,7 @@ def self.extended(model)
     #
     # @api public
     def determine_type(class_or_name)
-      @type_lookup_cache[class_or_name] ||= determine_type_and_cache(class_or_name)
+      @type_lookup_cache[class_or_name.to_s] ||= determine_type_and_cache(class_or_name)
     end
 
     # Return the default primitive supported