Crystal Report formula error

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C#

C#4.0

I am trying to create a formula like this(below) but it arises an error. I have mentioned the error here within double quotation mark
"A number is required here". This error is occured in the else portion
I want to show the normal date in the else portion. Please help me


if {Table1.ToDate}=""
then
month(datetime({Table1.FDate}))
else
datetime({Table1.FDate})