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

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 ···

Start in failpoint.go by tracing the Value definition and the parsing of return('100'), then reproduce the reported command with GO_FAILPOINTS and the demoPanic injection. Done means explaining why the runtime value is bool rather than string and aligning the documented behavior with the observed result if the implementation is incorrect.