It is NOT OK to just munmap memory from malloc :/.

apt-pkg/contrib/mmap.cc

@@ -307,10 +307,11 @@ DynamicMMap::~DynamicMMap()
       if (validData() == false)
 	 return;
 #ifdef _POSIX_MAPPED_FILES
-      munmap(Base, WorkSpace);
-#else
-      free(Base);
+      if ((Flags & Fallback) != Fallback) {
+         munmap(Base, WorkSpace);
+      } else
 #endif
+      free(Base);
       return;
    }