Harfbuzz throws an assert after upgrading to Qt4.6

After I upgraded to Qt4.6 my program started throwing an assert where it had been working fine in Qt4.5 and previous. I haven't tracked down what code causes the assert yet, but as I said my program had been working fine with all the previous versions of Qt4 going all the way back to Qt4.0.

program: /var/tmp/qt-x11-src-4.6.0/src/3rdparty/harfbuzz/src/harfbuzz-shaper.cpp:436: void HB_HeuristicSetGlyphAttributes(HB_ShaperItem*): Assertion `item->num_glyphs <= length' failed.