There is no contradiction if one views that there is no essential karma either.
In Mahayana philosophy the concept of selflessness is applied to both persons and phenomena.
Briefly, this view describes two levels of truth.
On the conventional level, I experience myself as though I really exist, I experience suffering, happiness, and a whole bunch of other things.
With detailed and thorough examination the sense of "I" as experienced is revealed to be delusional.
We can see through logic that the self cannot exist independently of mind and body, nor as the sum of the various parts of mind and body.
This is the selflessness of persons, if one applies the same logic to phenomena (for example a table) then the table is shown to be selfless (or tableless). This is the level of ultimate reality.
Both rebirth and karma are dependent upon notions of a self as you pointed out
On the ultimate level there is no self, table, rebirth or karma.
However, until this truth really penetrates and transforms us we really only experience the conventional level of reality.
It is in regards to conventional truth that karma and rebirth can be said to exist and effect us.
For me this distinction between two levels of truth is what really drew me to the Mahayana.
This distinction is what has facilitated my understanding of the majority of Buddhist teachings.
Because of my initial limited perceptions I could not understand a lot of what Buddhism teaches on the conventional level and had to put it on the back shelf (trying not to reject or accept too quickly, this was so hard for me!).
After about 5 or 6 years of putting karma & rebirth aside, the empirical evidence supporting them began to outweigh my doubts.
Once I began to let go of the worldview that was preventing me from being open to the possibility of rebirth & karma, things changed really quickly for me and I realized that I had been really closed minded! Prior to this I was having a really hard time with these aspects of BuddhaDharma and really did not believe that I would ever be able to relate to them.
The teachings on emptiness and repeated reflection of the relative nature of conventional truth have expanded my worldview immensely, without me needing to sacrifice logic or reasoning in any way.