CASE statement giving error in SSRS query designer

This is someting "back to basics" stuff.. :)

In SSRS query designer I needed write CASE statement like this:

        (CASE WHEN (tf_table.applicationtype = 1)  THEN ('first') 
         ELSE (tf_table.applicationtype) ) AS application

.....But I got error message:

Errors while loading the designer - fixed in code view

During these months when I have build reports with Reporting Services, I haven't got many error messages but here is one that I met today...

I got following error message when I tried to open report file in Reporting Services designer:

One or more errors encountered while loading the designer.--- The errors are listed below. --- Deserialization failed: The 'DataType' attribute is not declared. Line 4921, position 20.

I could open report file in code view by right-clicking report and choseing "view code". I scrolled to that part of the code which was metioned in error message; line 4921, position 20. Error was in prompt parametervalue command/rule: DataType= "Integer".
Line was: <Value DataType= "Integer">0</Value> 
I removed that text ' DataType="Integer" --> <Value>0</Value>

After this correction report could be open in design view.