reference #value error
 

I am using =COUNTIF(('[01.xls]Managers Page 1'!$D$19:$D$49),901), which looks
at 12 different workbooks that represent 12 months, and looking for a
specific code (the "901"), and trying to count the number of times it is used.

The problem is unless I have all the workbooks open, I get a #value error.
Am I using the wrong formula or is there something else I need to do?