Ignore blanks in sumproduct formula

I've found a simple formula to sum the total number of unique values in a column of data (invoice numbers), but have run into a spot of bother when there are blank cells at ANY point in the column. If all cells in the referenced range contain a value, it works perfectly. Does anyone know how i can modify the formula to ignore all blank cells? I've tried it as an array formula, but still no luck. Any ideas?

=SUMPRODUCT(1/COUNTIF(CB1:CB50,CB1:CB50))

Cheers!

Many Thanks,

It didn't quite work as it returned a decimal (0.3) when the outcome should have been 4. However you suggestion did make me look at the problem a different way and the result is this:

=IF(CB1:CB50="",0,SUM(SUMPRODUCT(1/COUNTIF(CB1:CB50,CB1:CB50&"")))-1)

And it works perfectly!!!

Thanks for your help!

NOTE! for my purposes, the minus 1 works as i will ALWAYS have a blank cell somewhere in the column. If a column is full with NO blanks, the formula will return an innacurate value (1 less than the true value).