using the expression =Sum([field]) returns #error, why?

[Guest]

i have a control on the footer of a form that is called [text30] , i want it
to caculate the values of a field called [dur], i used =Sum([field]) but it
returns #error although i usedit before and it was working fine!

what is the cause of this and hw to solve it?
thanks

 

[Douglas J. Steele]

If the field you're trying to sum is named [dur], you want =Sum([dur]).

Actually, if it's possible that one or more values in [dur] might be Null,
you're better off using =Sum(Nz([dur], 0))