diff --git a/spec/03-plugins/15-response-transformer/02-body_transformer_spec.lua b/spec/03-plugins/15-response-transformer/02-body_transformer_spec.lua
index 1d7ef34fdff..038cfb252df 100644
--- a/spec/03-plugins/15-response-transformer/02-body_transformer_spec.lua
+++ b/spec/03-plugins/15-response-transformer/02-body_transformer_spec.lua
@@ -138,8 +138,9 @@ describe("Plugin: response-transformer", function()
 
       it("plain text", function()
         local body = "test"
-        local json_body = body_transformer.transform_json_body(conf, body)
-        assert(json_body == nil)
+        local json_body, err = body_transformer.transform_json_body(conf, body)
+        assert.is_nil(json_body)
+        assert.equals(err, "failed parsing json body")
       end)
     end)