I am using this function on each line of an investment portfolio. Here
is the funtion as entered:

=RATE((TODAY()-C3)/365,0,I3,-K3)

C3 = Investment Date
I3 = Original Investment ammount
K3 = Current Value

The problem I'm having is it works on some lines and other lines it
returns #NUM

Here are 2 examples, the first I get a return of 122.5% and the 2nd
gives me #NUM.

C3=7/19/05, I3=162.00, K3=165.23 (returns 122.5%)
C3=7/19/05. I3=162.00, K3=153.76 (returns #NUM)

I don't understand, please help.