You are absolutely correct that this a problem with Yesod, in
particular Hamlet version 0.5. I specifically released version 0.6 to
make Cassius more flexible and to give better error messages. In
future versions of Yesod, Hamlet 0.5 will be excluded with a version
bound. Unfortunately, doing so during the Yesod 0.6 line would break
the PVP (I think).

Michael