Error swallowed when using ConstExpr with a custom func returning an error

Error is swallowed when using expr.ConstExpr(...) optimization with a custom function that has a signature which returns an error: func(i int) (int, error).

This example works correctly if expr.ConstExpr(...) setting is omitted, or optimizations are turned off. The error would be returned by expr.Run(...).

package main

import (
	"errors"
	"fmt"

	"github.com/antonmedv/expr"
)

func main() {
	env := map[string]interface{}{
		"double": func(i int) (int, error) {
			if i < 0 {
				return 0, errors.New("value cannot be less than zero")
			}
			return i * 2, nil
		},
	}

	options := []expr.Option{
		expr.Env(env),
		expr.ConstExpr("double"),
	}

	// err == nil here. Error is swallowed.
	compiledRule, err := expr.Compile("double(-1)", options...)
	if err == nil {
		fmt.Printf("expected expr.Compile to return error")
	} else {
		fmt.Println("success")
	}

	// err == nil here as well.
	_, err = expr.Run(compiledRule, env)
}