I hit the same issue, and I think the problem is that at line 515 in
pstats.py add_callers() the two stats instead of adding them
member-wise. As a result, each time add() is called, the number of stats
associated with each func grows by 4. Then, when print_call_line() is
called by print_callers() or print_callees(), there are "too many values
to unpack" at line 417.

This change to pstats.py modifies add_callers() to add the stats
together instead of concatenating the tuples.

515c515
<             new_callers[func] = caller + new_callers[func]
---
>             new_callers[func] = map(lambda x,y: x+y, caller +
new_callers[func])