variable assignments generate output in notebook

I'm having a somehow strange behaviour with the latest Toree kernel (checked out from github master branch). Every assignment on a cell generates notebook output, as it can be seen in the attached figure

The output produces the value of the variable (but apparently not at the time of assignment, but the final value in the cell). I see no means of switching that off (I've tried to find a %%magic to activate/deactivate it, but found no one). Is that expected behaviour? If I try an earlier version of the kernel on the same system, no output is generated (except the one explicitly generated by printing, or the usual last value in the cell)