ByRef Argument type mismatch with Boolean

I have some vba code in an Access form which produces a "ByRef Argument type mismatch" error when called under the following circumstances. I have a small function ``` NullAndHide(ctl as control,displayitem as Boolean) ``` which works as expected when I call it like so. ``` Call NullAndHide(Me.Control,True) ``` However, if I use the following case statement to try to set the value of displayitem based on another control: ``` Dim PerPersonOption, PerRoomOption As Boolean Select Case PriceType_ID Case Is = 1 'Per Person PerPersonOption = True PerRoomOption = False Case Is = 2 'Per Room PerPersonOption = False PerRoomOption = True End Select ``` And then `Call NullAndHide(Me.Control,PerPersonOption)` I get the error: ByRef Argument type mismatch I've tested the the value of PerPersonOption with ``` msgBox PerPersonOption ``` and it returns the correct boolean value. My function expects a Boolean, I'm giving it a Boolean - So why am I getting this error?