Fehler bei Feature Suche

Query: curl http://localhost:3000/api/features\?category\=m``an

Fehler:

TypeError: Cannot read property 'trim' of undefined
    at countReferences (/home/markus/github/vue-letters/lib.js:34:51)
    at searchFeaturesInDocument (/home/markus/github/vue-letters/lib.js:128:25)
    at Object.features (/home/markus/github/vue-letters/lib.js:162:12)
    at /home/markus/github/vue-letters/app.js:190:25
    at Layer.handle [as handle_request] (/home/markus/github/vue-letters/node_modules/express/lib/router/layer.js:95:5)
    at next (/home/markus/github/vue-letters/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/home/markus/github/vue-letters/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/home/markus/github/vue-letters/node_modules/express/lib/router/layer.js:95:5)
    at /home/markus/github/vue-letters/node_modules/express/lib/router/index.js:281:22
    at param (/home/markus/github/vue-letters/node_modules/express/lib/router/index.js:354:14)