Kirill is right; tpd doesn't work under the union type. The errors look like this: /home/hniksic/work/emacs/xemacs-20-hack/src/alloc.c:4329: wrong type argument to unary exclamation mark And indeed: if (!backtrace[i].obj) The `obj' element of backtrace structure is a Lisp object, so unary ! won't work under union type. To be compatible with union type, you probably want to use: if (LISP_TO_VOID (backtrace[i].obj)) Similar mistakes exist in other places in alloc.c.