When running the following expression, it returns in the error interface conversion: interface {} is nil, not string (1:9)

We would like to handle this gracefully, as the query parameter are optional that we're matching on. We prefer not to wrap it in string(), as our customer may create their own expression, and prefer to have this expression as clean as possible.

Is there any way to combat this, with for instance AllowUndefinedVariables? Or can expr-lang be modified to allow contains queries etc. on possible nil types?

env := map[string]interface{}{
	"query": map[string]interface{}{
		"another_query": "test",
	},
}

expression := "query.b contains 'test'"
prg, _ := expr.Compile(expression, expr.Env(env))
result, err := expr.Run(prg, env)
fmt.Println(result)
fmt.Println(err)

Output:

interface conversion: interface {} is nil, not string (1:9)
 | query.b contains 'test'