GetSheetIndex returns -1

[AngangGuo]

Excelize v2.3.1

func (f *File) GetSheetIndex(name string) int
GetSheetIndex provides a function to get a sheet index of the workbook by the given sheet name. 
If the given sheet name is invalid, it will return an integer type value 0. 

The method returns -1 if the sheet name is invalid or if the sheet name doesn't exist. The last sentence should be:

If the given sheet name is invalid or sheet doesn't exist, it will return an integer type value -1.