Trouble with System plugin

I'm trying to get information about the styles used by the installer window. Executing the following in a page show event:

!define GWL_STYLE -16

System::Call "user32:GetWindowLong(i, i) i ($HWNDPARENT, ${GWL_STYLE}) .s"

Pop $R0

MessageBox MB_OK `"$R0"`

The message box always shows ""

What am I doing wrong?

EDIT:
Apologies, have been looking at this for hours, finally gave up and posted, then noticed that there is only one colon in the method call. D'oh!

!define GWL_STYLE -16

System::Call "user32::GetWindowLong(i, i) i ($HWNDPARENT, ${GWL_STYLE}) .s"

Pop $R0

MessageBox MB_OK `"$R0"`

^^this works fine.