diff --git a/simulate/simulate.cc b/simulate/simulate.cc
index cb8bd4d6..d25362e3 100644
--- a/simulate/simulate.cc
+++ b/simulate/simulate.cc
@@ -1633,7 +1633,7 @@ void UiEvent(mjuiState* state) {
   // 3D scroll
   if (state->type==mjEVENT_SCROLL && state->mouserect==3) {
     // emulate vertical mouse motion = 2% of window height
-    if (!sim->is_passive_) {
+    if (sim->m_ && !sim->is_passive_) {
       mjv_moveCamera(sim->m_, mjMOUSE_ZOOM, 0, -zoom_increment*state->sy, &sim->scn, &sim->cam);
     } else {
       mjv_moveCameraFromState(