[Fedora-packaging] problem with scratch build or mock

Hi, I made the src.rpm under F11 then koji build --scratch dist-5E-epel /home/philippe/rpmbuild/SRPMS/firebird-2.1.3.18156.0-0.fc11.src.rpm can someone explain me why I get this : DEBUG util.py:256: error: unpacking of archive failed on file /builddir/build/SOURCES/Firebird-2.1.3.18156-0.tar.bz2;4a3bf995: cpio: MD5 sum mismatch and even using mock -r epel-5-i386 rebuild ./SRPMS/firebird-2.1.3.18156.0-0.fc11.src.rpm on my box I get into the same trouble