diff --git a/openmmapi/src/PlumedForceImpl.cpp b/openmmapi/src/PlumedForceImpl.cpp
index 0752e2f..2eaee4b 100644
--- a/openmmapi/src/PlumedForceImpl.cpp
+++ b/openmmapi/src/PlumedForceImpl.cpp
@@ -139,6 +139,7 @@ double PlumedForceImpl::computeForce(ContextImpl& context, const vector<Vec3>& p
 
     // Calculate the forces and energy.
 
+    memset(&forces[0], 0, forces.size()*sizeof(Vec3));
     plumed_cmd(plumedmain, "prepareCalc", NULL);
     plumed_cmd(plumedmain, "performCalcNoUpdate", NULL);
     if (step != lastStepIndex) {