Custom White Balance Error?

[flow]

I wanted to use the custom WB option the other day, but was not able to. I had tested it at home the night before and it worked fine ... but trying it repeatedly when I was shooting, I kept getting an error message that it couldn't set it. ("Unable to measure preset white balance. Please try again.") I was holding a white index card right in front of the lens, right where I was shooting. I eventually gave up and just picked one of the other settings, but would like to know what I did wrong? Or is this a common issue? If it happens again, is there a get-around?
(This was with a D40.)

 

[KmH]

See page 108 - Troubleshooting - of your D40 user's manual.

Problem: Cannot measure White Balance. Solution: Subject is to dark or to bright Reference: page 71

D40 user's guide download at - Nikon Product Manuals available for download

 

[Joves]

Unless you have enough light then it will not measure it. Your only other option is to set the WB by using Temperature. I have done this when I could not get a measurement with my D300 for street lighting. In that case the lighting at hand was in the 2500K range, not exactly but close enough.