It also violates the requirement that the vote may not explicitly run counter to the intention of the voter.
[1] The negative vote weight could occur in a state where a party had overhang seats.
In such a state, in many circumstances a vote for a given party would not increase its national seat total (MMP allocations are calculated at the national level in Germany), but might increase the proportion of the party's national seats to be allocated to the state where the vote was cast and which already had overhang seats (overhang seats being allocated at the state level).
The Federal Constitutional Court of Germany ruled on 3 July 2008 that this was unconstitutional, as a negative vote weight is incompatible with the constitutional guarantee of equality and directness.
[2][3] Negative vote weight is possible in national elections in the Czech Republic and Romania.