Can't call internal tracer in `traceBlockByNumber`

[lightclient]

System information

Geth version: 1.9.25
OS & Version: Ubuntu

Expected behaviour

Run internal tracers.

Actual behaviour

Error: json: error calling MarshalJSON for type json.RawMessage: unexpected end of JSON input
        at web3.js:6347:37(47)
        at web3.js:5081:62(37)
        at <eval>:1:25(7)

Steps to reproduce the behaviour

From JS console:

> debug.traceBlockByNumber(11701516, {tracer: 'unigramTracer'})