Formula Audit Problem
 

When I try to use the evaluate formula auditing on this formula, it bombs out
on the first eval, but the formula works just fine. Any ideas why it'd bomb
rather than evaluate?
=IF(W5<$K$1,100,IF(ISERROR(YIELDMAT($K$1,W5,$D5,$B 5,($J5+$K5)/2)),100,IF(YIELDMAT($K$1,W5,$D5,$B5,($J5+$K5)/2)<0,100,YIELDMAT($K$1,W5,$D5,$B5,($J5+$K5)/2))))

W5 = date that may be before tomorrow
K1 = tomorrow
Other stuff is bond yield inputs

TIA