diff --git a/lib/graphiti/util/serializer_relationships.rb b/lib/graphiti/util/serializer_relationships.rb
index 5ab52b2e..70bf886d 100644
--- a/lib/graphiti/util/serializer_relationships.rb
+++ b/lib/graphiti/util/serializer_relationships.rb
@@ -118,7 +118,7 @@ def validate_link_for_sideload!(sideload)
         cache_key = :"#{@sideload.object_id}-#{action}"
         return if self.class.validated_link_cache.include?(cache_key)
         prc = Graphiti.config.context_for_endpoint
-        unless prc.call(sideload.resource.endpoint[:full_path], action)
+        unless prc.call(sideload.resource.endpoint[:full_path].to_s, action)
           raise Errors::InvalidLink.new(@resource_class, sideload, action)
         end
         self.class.validated_link_cache << cache_key