Why do I get 'Invalid procedure call or argument'?

This often happens because you used a VBScript "friendly name" constant in place of its integer equivalent. Visual Basic understands these friendly names, such as FileSystemObject's 'forAppending' and 'forWriting' constants. 
 
An easy solution is to add this line to any include files you use in every page (or else add it at the top of every page, if you don't have a common header in your application): 
 

<%      Const ForReading = 1, ForWriting = 2, ForAppending = 8  %>

 

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Another possible cause is trying to use a string operation (like Mid(), InStr(), Left() or Right()) on a NULL value. So for example: 
 

<%      response.write Left(rs("column"), 10)  %>

 
Should be: 
 

<%      if len(rs("column")) > 0 then          response.write Left(rs("column"), 10)      else          response.write " "      end if  %>

 
If you're having these problems with NULL values coming out of a database, see Article #2150. 
 

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Another possibility is that you are coming over from JavaScript, or otherwise think that string lengths are 0-based. The following code sample will cause this error: 
 

<%      str = "foo"      response.write Mid(str, 0, 1)    %>

 
Similarly, passing a starting argument of 0 to the Instr() function will cause the same error. To solve this problem, always start at 1 for string parsing in VBScript. 
 

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Still one more possibility is that you have unexpected characters in an FSO writeline call. I recently came across this when using MSXML to write out contents of dynamic ASP files into static HTML files. I had this: 
 

<%      set fs = fso.CreateTextFile("blat.htm", true)      fs.writeline xmlHTML  %>

 
When I ran this code, I received the above error. When I added the unicode format designator, as follows, everything worked fine: 
 

<%      set fs = fso.CreateTextFile("blat.htm", true, -1)      fs.writeline xmlHTML  %>