how can i solve this error?, snappy usage

I've tried to find a way to solve this error, but haven't found it yet

```Declaration of Barryvdh\Snappy\IlluminateSnappyPdf::getFileContents($filename) must be compatible with Knp\Snappy\AbstractGenerator::getFileContents(string $filename): string```

above there is an error that i can't to solve

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