Misbehaving git bisect bad HEAD

Running "git bisect bad" should be the same as "git bisect bad HEAD", shouldn't it?

When replaying this bisect log on the Linux kernel tree:

git bisect start # bad: [97bf6af1f928216fd6c5a66e8a57bfa95a659672] Linux 3.19-rc1 git bisect bad 97bf6af1f928216fd6c5a66e8a57bfa95a659672 # good: [b2776bf7149bddd1f4161f14f79520f17fc1d71d] Linux 3.18 git bisect good b2776bf7149bddd1f4161f14f79520f17fc1d71d # good: [70e71ca0af244f48a5dcf56dc435243792e3a495] Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next git bisect good 70e71ca0af244f48a5dcf56dc435243792e3a495 # good: [988adfdffdd43cfd841df734664727993076d7cb] Merge branch 'drm-next' of git://people.freedesktop.org/~airlied/linux git bisect good 988adfdffdd43cfd841df734664727993076d7cb # good: [b024793188002b9eed452b5f6a04d45003ed5772] staging: rtl8723au: phy_SsPwrSwitch92CU() was never called with bRegSSPwrLvl != 1 git bisect good b024793188002b9eed452b5f6a04d45003ed5772 # good: [66dcff86ba40eebb5133cccf450878f2bba102ef] Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm git bisect good 66dcff86ba40eebb5133cccf450878f2bba102ef # bad: [88a57667f2990f00b019d46c8426441c9e516d51] Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip git bisect bad 88a57667f2990f00b019d46c8426441c9e516d51 # good: [0ec28c37c21a2b4393692e832e11a7573ac545e2] Merge tag 'media/v3.19-2' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media git bisect good 0ec28c37c21a2b4393692e832e11a7573ac545e2 # good: [c0f486fde3f353232c1cc2fd4d62783ac782a467] Merge tag 'pm+acpi-3.19-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm git bisect good c0f486fde3f353232c1cc2fd4d62783ac782a467 # bad: [34b85e3574424beb30e4cd163e6da2e2282d2683] Merge tag 'powerpc-3.19-2' of git://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux git bisect bad 34b85e3574424beb30e4cd163e6da2e2282d2683 # good: [64ec45bff6b3dade2643ed4c0f688a15ecf46ea2] Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost git bisect good 64ec45bff6b3dade2643ed4c0f688a15ecf46ea2

Running "git bisect bad" gives this:

$ git bisect bad 
Bisecting: 6 revisions left to test after this (roughly 3 steps)
[ec2aef5a8d3c14272f7a2d29b34f1f8e71f2be5b] power/perf/hv-24x7: Use kmem_cache_free() instead of kfree

Running "git bisect bad HEAD" instead gives this:

$ git bisect bad HEAD
Bisecting: a merge base must be tested
[56548fc0e86cb9156af7a7e1f15ba78f251dafaf] powerpc/powernv: Return to cpu offline loop when finished in KVM guest

This is git 2.2.1.

Andreas.