Lua error.

If the addon's window is NOT showing when I enter the world and if my addons are sending messages to this addons windows, I get the following error when I bring up this addons window and try to scroll (also, no data is showing until I try to scroll. I get the error below and then the data shows):

2x SharedXML\HybridScrollFrame.lua:67: attempt to perform arithmetic on local 'stepSize' (a nil value)
SharedXML\HybridScrollFrame.lua:67: in function <SharedXML\HybridScrollFrame.lua:57>

.... (a lot more, but I didn't paste it all)