diff --git a/lib/jsonapi/request.rb b/lib/jsonapi/request.rb
index bbd42961c..446dfde9c 100644
--- a/lib/jsonapi/request.rb
+++ b/lib/jsonapi/request.rb
@@ -584,8 +584,8 @@ def parse_update_relationship_operation(verified_params, relationship, parent_ke
       if relationship.is_a?(JSONAPI::Relationship::ToOne)
         if relationship.polymorphic?
           operation_args[1].merge!(
-            key_value: verified_params[:to_one].values[0][:id],
-            key_type: verified_params[:to_one].values[0][:type]
+            key_value: verified_params[:to_one].values[0] && verified_params[:to_one].values[0][:id],
+            key_type: verified_params[:to_one].values[0] && verified_params[:to_one].values[0][:type]
           )
 
           operation_klass = JSONAPI::ReplacePolymorphicToOneRelationshipOperation