-f option results in invalid index

[redneb]

When I create a tpxz archive as follows

$ pixz -9 -f 3 linux-3.7.4.tar linux-3.7.4.tpxz

then the resulting archive has a corrupt index:

$ pixz -x linux-3.7.4/README < linux-3.7.4.tpxz > /dev/null 
Index and archive differ as to next file: linux-3.7.4/README vs linux-3.7.4/

This seems to happen for all values of -f greater than 2. I've only tested this with the -9 compression level.