custom code not working to check if the field is missing or not

Hello All,

                  In my ssrs report I need to check if the field is missing or not as my stored procedure returns varying number of columns based on input parameter.

This is the code I am using in report properties under Code

Public Function IsFieldMissingThenString(field as Field, strMessage as String) as String
 If (field.IsMissing) Then
  Return strMessage
 Else 
  Return field.Value
  End If
End Function

I am calling this method inside the table as below

=Code.IsFieldMissingThenString(Fields!Yarnsize.Value,"Missing")

But I am still getting the error '#Error'

Please let me know if I am missing something here.