Why is Newrelic Ruby Agent raising "Unexpected token < in JSON"

Question

I'm getting a strange error in a Rails app

Uncaught SyntaxError: Unexpected token < in JSON at position 0 new:7

The browser console is showing this error as being raised by code injected by newrelic ruby agent.

Line 7: <script>(window.NREUM||(NREUM={})).loader_config={xpid:"xxxxxxx"};window.NREUM||(NREUM={}),__nr_require.........#and so on

The newrelic code appears on every page, but the error only seems to occur on one page.

This page is consuming a couple of internal JSON APIs. One creates a remote Select2 dropdown from a countries json feed, and another renders a Mapbox map from the same feed of countries.

The API is formatted as GeoJSON (with no unexpected < tokens!)

[
  {
    "type":"Feature",
    "geometry":{
      "type":"Point",
      "coordinates":[0,0]
     },
     "properties": {
       # some attributes
     }
  }
]

Why is the NewRelic code interacting with JSON? And what is a good strategy or approach for debugging this? As the error is being raised by code injected by the Newrelic Ruby Agent, I'm unclear where to begin looking to resolve this.

Answer 1

I can't speak too this situation specifically, but historically I have noticed this type of error happens when a resource is not found, and an HTML page is returned in place of the resource (a 404 page or something similar).