Metric groups attached to biextensions

Let $G$ be a connnected, unipotent, perfect group scheme over an algebraically closed field of characteristic p > 0. V. Drinfeld has defined a certain metric group associated to biextensions of $G \times G$ by the discrete group $Q_p/Z_p$. We prove a certain conjecture of Drinfeld regarding the class of this metric group in the Witt group.