Observed in Beta. See: https://cxserver-beta.wmflabs.org/v1

Stack trace:

{
  "name": "cxserver",
  "hostname": "deployment-sca01",
  "pid": 14,
  "level": 50,
  "message": "500: internal_error",
  "stack": "TypeError: Cannot read property key of undefined\n    at /srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/src/app.js:126:56\n    at Layer.handle [as handle_request] (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/layer.js:95:5)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:137:13)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:131:14)\n    at Route.dispatch (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/route.js:112:3)\n    at Layer.handle [as handle_request] (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/layer.js:95:5)\n    at /srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/index.js:281:22\n    at param (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/index.js:354:14)\n    at param (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/index.js:365:14)\n    at Function.process_params (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/index.js:410:3)\n    at next (/srv/deployment/cxserver/deploy-cache/revs/a8e7abf16ba896656ba4f144b96691a5ba9a5a06/node_modules/express/lib/router/index.js:275:10)",
  "status": 500,
  "type": "internal_error",
  "detail": "Cannot read property key of undefined",
  "request_id": "d52fd902-0423-11e9-9897-3a6e8d205aec",
  "request": {
    "url": "/favicon.ico",
    "headers": {
      "user-agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36",
      "x-request-id": "d52fd902-0423-11e9-9897-3a6e8d205aec"
    },
    "method": "GET",
    "params": {
      "0": "/favicon.ico"
    },
    "query": {},
    "remoteAddress": "10.68.21.68",
    "remotePort": 47674
  },
  "levelPath": "error/500",
  "msg": "500: internal_error",
  "time": "2018-12-20T06:52:35.857Z",
  "v": 0
}