Mercury merge of two named branches: working directory has no ancestor

I have two branches with a name, SPRINT_009 and SPRINT_010. Some changes were dedicated to SPRINT_009, which I would like to combine into SPRINT_010. I was able to easily change the set of changes from branch_one back to the default value. I'm having problems merging change sets from branch_one to branch_two.

default       A -- B -- C -- G -- H -- I -- J -- K -- L -- N -- O -- P
                         \                             \            /
SPRINT_009     D -- E -- F -----------------  ---------- ----------
                                                         \
SPRINT_010                                                M

Here are the commands I use:

hg update branch_two
hg merge branch_one

However, I get the following message:

abort: merging with a working directory ancestor has no effect

Does anyone know what exactly I'm doing wrong?

** EDIT: I added a graphic image. Figure conceptual **