Input string was not in a correct format

[Aki]

ho questo codice:

If Page.IsValid then
if a <> "" and b <> "" and c > 0 and d <> "" and e > 1950 then 
   .....
end if
end if

facendo scatenare l'evento, mi dà:

Server Error in '/wcv' Application.
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Input string was not in a correct format.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.FormatException: Input string was not in a correct format.