All-black stacked image, error doing two-color transform

I have been processing many sets of BVRI images of VY Aqr from BSM and have run into a snag. I'm getting 2-3 images in each filter so I've been stacking (average) each filter set before doing a two-color transformation. One such set, a pair of B images from BSM_Berry2, produces what looks like an all-black image when stacked. However, I can load the sequence I'm using and VPHOTO will happily match up the invisible stars - clicking on one of the stars and selecting "Details" shows a normal-looking brightness plot for the star. Since there were obviously stars on the image, and it appeared to be only a display problem, I tried to do a two-color transform on the B and V stacked images and got this error:

[ArgumentException: Argument 'Expression' is not a valid value.]
Microsoft.VisualBasic.Strings.Format(Object Expression, String Style) +768
Restricted_ColorPhotReport.CreateCompGrid(String bcolorBlue, String bcolorRed, String imgURL_B, String imgURL_R, Double meanB, Double meanR, Double meanCI, Double stdB, Double stdR, Double stdCI, String targetname) +2470
Restricted_ColorPhotReport.ShowReport() +1997
Restricted_ColorPhotReport.CreateReport(Int32 seqID, Double Tbr, Double Tb, Double Tr, Int32 ap, Int32 minSNR, Int32 searchRad, Single initFWHM, Int16 filterBlue, Int16 filterRed, Int64 imgID_B, Int64 imgID_R, Double factor, Boolean useFactor, Hashtable compsToExclude) +4322
Restricted_ColorPhotReport.Page_Load1(Object sender, EventArgs e) +821
System.Web.UI.Control.OnLoad(EventArgs e) +95
System.Web.UI.Control.LoadRecursive() +59
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +678

Shawn (DKS)