Cannot filter messages by a json format

[ifat-afek]

Describe the bug
I have a log file that contains both text strings and json-formatted log messages.
I tried to use router or filter in order to filter only the json messages but it didn't work. I followed the example in https://github.com/observIQ/stanza/blob/master/docs/operators/router.md

To Reproduce
I used the following pipeline:
pipeline:

• id: message_type_router
  type: router
  routes:
  • output: message_router
    expr: '$.format == "json"'
• id: message_router
  type: router
  routes:
  • output: stdout
    expr: '$record.log != nil and $record.log.message != nil and $record.log.message matches "XYZ"

Expected behavior
Json formatted messages should be passed to message_router and then checked against the expression (that fails for regular strings).

Actual behavior
I get errors in the log:
{"level":"warn","timestamp":"2022-01-02T16:28:05.671Z","message":"Running expression returned an error","operator_id":"$.message_type_router","operator_type":"router","error":"invalid operation: int(string) (1:3)\n | $.format == "json"\n | ..^"}

Environment:

• Running on k8s
• Stanza Version 1.4.2