Allow only values that can be serialized to key (#6881)
Allow only values that can be serialized to key (#6881)

@ijjk
azukaru/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js,BlancheXu/test,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,BlancheXu/test,flybayer/next.js,zeit/next.js,BlancheXu/test,azukaru/next.js,azukaru/next.js,flybayer/next.js