merge in `go-jmespath` / `jp` panics when parameters are not JSON objects

[n9]

echo {"message": "foo"} | jp "merge({message: message}, contextMap)"

panic: interface conversion: interface {} is nil, not map[string]interface {}

goroutine 1 [running]:
github.com/jmespath/go-jmespath.jpfMerge({0xc00004ec20, 0x2, 0xc00000e3c0})
        /Users/jamessar/Source/go/pkg/mod/github.com/jmespath/go-jmespath@v0.4.0/functions.go:520 +0x197
github.com/jmespath/go-jmespath.(*functionCaller).CallFunction(0x70e1c0, {0xc00000e3c0, 0x5}, {0xc00004ec20, 0x2, 0x2}, 0xc000006060)
        /Users/jamessar/Source/go/pkg/mod/github.com/jmespath/go-jmespath@v0.4.0/functions.go:401 +0x264
github.com/jmespath/go-jmespath.(*treeInterpreter).Execute(0xc000006060, {0x4, {0x707500, 0xc00004cfd0}, {0xc000020420, 0x2, 0x2}}, {0x711c40, 0xc0000769f0})
        /Users/jamessar/Source/go/pkg/mod/github.com/jmespath/go-jmespath@v0.4.0/interpreter.go:77 +0x1f35
github.com/jmespath/go-jmespath.Search({0xc00000e3c0, 0x25}, {0x711c40, 0xc0000769f0})
        /Users/jamessar/Source/go/pkg/mod/github.com/jmespath/go-jmespath@v0.4.0/api.go:48 +0x173
main.runMain(0xc000154000)
        /Users/jamessar/Source/jp/jp.go:105 +0x7d4
main.runMainAndExit(0xc000072a00)
        /Users/jamessar/Source/jp/jp.go:51 +0x19
github.com/urfave/cli.HandleAction({0x706800, 0x7482c0}, 0x25)
        /Users/jamessar/Source/go/pkg/mod/github.com/urfave/cli@v1.22.5/app.go:526 +0x50
github.com/urfave/cli.(*App).Run(0xc000152000, {0xc00004e3c0, 0x2, 0x2})
        /Users/jamessar/Source/go/pkg/mod/github.com/urfave/cli@v1.22.5/app.go:286 +0x625
main.main()
        /Users/jamessar/Source/jp/jp.go:47 +0x4a5

[b-enoit-be]

The behaviour I would expect when doing a merge(`{"a": 1}`, null) would be to get {"a": 1} as a result, as null, is a valid JSON object.

Or at least it works like that on the site: https://jmespath.org