JSON.parse failure on first benchmark run?

Hi, I'm trying to setup benchmarks for a project of mine (PR can be found here: https://github.com/luc-tielen/eclair-lang/pull/128)

I tried a lot of different things, but I keep getting the following error: `Error: Output file for 'custom-(bigger|smaller)-is-better' must be JSON file containing an array of entries in BenchmarkResult format: Unexpected token ' in JSON at position 0`.

It looks like the code can't read the previous benchmark result or fails to parse it as JSON. The output that's written to the benchmark file is a valid JSON array though..