This is wrong...I think

[cbwaters]

I have a Vivitar 2x PK-A/R-PK TC.  When I hook it up to a lens and my 
*ist-D, the camera won't display the aperture size in the viewfinder.  For 
example, with just the lens, I get f4.5, lens +TC and I get F--

Is my TC screwed-up of is this just an oddity?
CW