Zip task fails when a sub dir entry is added

[Joe V]

I have an item collection like below. When creating a zip using Zip task, it is failing with error "error : An item with the same key has already been added".

c:\project\package\test
c:\project\package\tools
c:\project\package\docs
c:\project\package\docs\license
c:\project\package\test\test1.results

The zip task seem to add c:\project\package\test, c:\project\package\tools,c:\project\package\docs fine, but is failing when it is trying to add sub dir c:\project\package\docs\license. These are all directory entries.

What is the trick to get this working?