[CMake] JOIN generator expression not working

[Daniel Dilts]

I have this generator expression:

-I$<JOIN:$<TARGET_PROPERTY:${LibraryName},INCLUDE_DIRECTORIES>, -I>

I get this output in my custom command:

-I$<JOIN:$ -I>

This is almost exactly the same as the example in the documentation (other
than adding ${LibraryName}).  Why doesn't this work?
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