Assertion error while synthesizing in Quartus2 9.0

I got a similar error: (.... protects the innocent).

Error: MGL_INTERNAL_ERROR: Assertion failure inside rtl_body at

/build/swbuild/SJ/nightly/9.1sp1/304/l32/p4/quartus/.....cpp:1964

CAUSE : The condition in MGL_ASSERT () failed at...

Error: Assertion error: The current megafunction is configured for use

with the clear box feature and cannot be used when the clear box

feature is disabled File:

..../9.1/quartus/libraries/megafunctions/altmult_add.tdf Line: 1540

Turns out I had generated some IP (in this case the SOPC) using

Stratix IV, but was targeting the top at a Stratix II. Bogus move for

sure, but the help message could've been better.

Getting the families to line up fixed the error.