create function and apply it in datavalidation custom formula

hi my friends , this is my function to to check space
it is my function code :

Function check_space(str) As Boolean
Dim i As Integer
Dim asci_code As Integer
For i = 1 To Len(str)
asci_code = Asc(Mid(str, i, 1))
If asci_code = 32 Then
remove_space = True
Else
remove_space =False
End If
Next
End Function


now i want use my function check_space , in datavalidation custom box by typing this formula :
check_space(a1)
it cant work...
plz help