Kernel bug hunt - 'Commit bisect start' yields 'A merge base must be tested'

[John Hupp]

I'm trying to track down a kernel bug, and I'm just starting to commit 
bisect the Ubuntu kernels as detailed at 
https://wiki.ubuntu.com/Kernel/KernelBisection#Commit_bisecting_Ubuntu_kernel_versions

However, when I run the 'git bisect start <BAD> <GOOD>' command, it 
responds with 'Bisecting: a merge base must be tested'

This message is explained lightly in the same article's FAQ at 
https://wiki.ubuntu.com/Kernel/KernelBisection#Bisecting:_a_merge_base_must_be_tested

But it doesn't actually say how to test the identified commit. Since 
this is the first result, there is no debian.master/changelog yet (as 
far as I know) to edit as instructed at 
https://wiki.ubuntu.com/Kernel/KernelBisection#Give_this_test_a_version_number. 
And has git also reset the tree so that the identified merge base commit 
is now the top commit?

Can someone point the way here?