Why the type of failpoint.Value can't be string

[SmartMalphite]

the definition in the source code about Value's type in "github.com/pingcap/failpoint/failpoint.go"

	// Value represents value that retrieved from failpoint terms.
	// It can be used as following types:
	// 1. val.(int)      // GO_FAILPOINTS="failpoint-name=return(1)"
	// 2. val.(string)   // GO_FAILPOINTS="failpoint-name=return('1')"
	// 3. val.(bool)     // GO_FAILPOINTS="failpoint-name=return(true)"
	Value interface{}

but in my code

failpoint.Inject("demoPanic", func(val failpoint.Value) {
		fmt.Println(val)
		fmt.Println(val.(string))
}

and start my progrom with cmd:
GO_FAILPOINTS="mypackagename/demoPanic=return('100')" ./beego-backend
An exception was thrown
[panic.go:522] Handler crashed with error interface conversion: failpoint.Value is bool, not string ···

[amyangfei]

ref to: #61

Should use either GO_FAILPOINTS="mypackagename/demoPanic=return(\"100\")" or GO_FAILPOINTS='mypackagename/demoPanic=return("100")'

[jinjiaKarl]

// add.go
func add(ctx context.Context) string {
     failpoint.InjectContext(ctx, "failpoint-name1", func(value failpoint.Value) string {
         failpoint.Return(value.(string))
     })

     return "1"
 }
// add_test.go
 func TestAdd(t *testing.T) {
     err := failpoint.Enable("golangtest/gofailpoint/failpoint-name1", "return('0.01')")
     if err != nil {
         log.Fatal(err)
     }
failpoints := map[string]struct{}{
         "golangtest/gofailpoint/failpoint-name1": {},
     }
     ctx := failpoint.WithHook(context.Background(), func(ctx context.Context, fpname string) bool {
         _, found := failpoints[fpname]
         return found
     })
     log.Println(add(ctx))
 }

when I execute my program, it truns out that:

error on golangtest/gofailpoint/failpoint-name1: failpoint: failed to parse "return('0.01')" past ")"

how come does not it support "return('0.01')" of this pattern?