Can any one solve this?..Type mismatch: 'RegisteruserFunc' error message while executing vbs

public function verifybutton(a)

b=a.getroproperty(a)

if b=true then

msgbox "Enabled"
else
msgbox "Disabled"

end if

end function

registeruserfunc "Webbutton","verifybutton","verifybutton",True

while running this it is throwing below attached error message

Error: Type mismatch: 'RegisteruserFunc'
code: 800A000D
source: Microsoft VBScript runtime error