diff --git a/src/engine/engine_io.c b/src/engine/engine_io.c
index e64d836f..bdedbaab 100644
--- a/src/engine/engine_io.c
+++ b/src/engine/engine_io.c
@@ -1334,11 +1334,11 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
 #endif
 
 #ifdef MEMORY_SANITIZER
-  // Tell msan to treat the entire buffer as uninitialized
+  // under MSAN, mark the entire buffer as uninitialized
   __msan_allocated_memory(d->buffer, d->nbuffer);
 #endif
 
-  // zero out arrays that are not affected by mj_forward
+  // zero out user-settable state and input arrays (MSAN: mark as initialized)
   mju_zero(d->qpos, m->nq);
   mju_zero(d->qvel, m->nv);
   mju_zero(d->act, m->na);
@@ -1346,11 +1346,10 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
   for (int i=0; i < m->neq; i++) d->eq_active[i] = m->eq_active0[i];
   mju_zero(d->qfrc_applied, m->nv);
   mju_zero(d->xfrc_applied, 6*m->nbody);
-  mju_zero(d->qacc, m->nv);
+  mju_zero(d->qacc, m->nv);  // input to inverse dynamics
   mju_zero(d->qacc_warmstart, m->nv);
   mju_zero(d->act_dot, m->na);
   mju_zero(d->userdata, m->nuserdata);
-  mju_zero(d->sensordata, m->nsensordata);
   mju_zero(d->mocap_pos, 3*m->nmocap);
   mju_zero(d->mocap_quat, 4*m->nmocap);
 
@@ -1359,7 +1358,7 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
 
   // copy qpos0 from model
   if (m->qpos0) {
-    memcpy(d->qpos, m->qpos0, m->nq*sizeof(mjtNum));
+    mju_copy(d->qpos, m->qpos0, m->nq);
   }
 
   // set mocap_pos/quat = body_pos/quat for mocap bodies
diff --git a/src/engine/engine_sensor.c b/src/engine/engine_sensor.c
index 42f9873c..c218a1dc 100644
--- a/src/engine/engine_sensor.c
+++ b/src/engine/engine_sensor.c
@@ -647,6 +647,8 @@ void mj_sensorPos(const mjModel* m, mjData* d) {
         break;
 
       case mjSENS_USER:                                   // user
+        // clear result, compute later
+        mju_zero(d->sensordata + adr, m->sensor_dim[i]);
         nusersensor++;
         break;
 
@@ -822,6 +824,8 @@ void mj_sensorVel(const mjModel* m, mjData* d) {
         break;
 
       case mjSENS_USER:                                   // user
+        // clear result, compute later
+        mju_zero(d->sensordata + adr, m->sensor_dim[i]);
         nusersensor++;
         break;
 
@@ -1346,6 +1350,8 @@ void mj_sensorAcc(const mjModel* m, mjData* d) {
         break;
 
       case mjSENS_USER:                                   // user
+        // clear result, compute later
+        mju_zero(d->sensordata + adr, m->sensor_dim[i]);
         nusersensor++;
         break;
 
diff --git a/test/engine/engine_sensor_test.cc b/test/engine/engine_sensor_test.cc
index c5607223..ab43f1e4 100644
--- a/test/engine/engine_sensor_test.cc
+++ b/test/engine/engine_sensor_test.cc
@@ -69,7 +69,6 @@ using SensorTest = MujocoTest;
 
 // --------------------- test sensor disableflag  ------------------------------
 
-// hand-picked positions and orientations for simple expected values
 TEST_F(SensorTest, DisableSensors) {
   constexpr char xml[] = R"(
   <mujoco>
@@ -83,7 +82,8 @@ TEST_F(SensorTest, DisableSensors) {
   ASSERT_THAT(model, NotNull()) << error;
   mjData* data = mj_makeData(model);
 
-  // before calling anything, check that sensors are initialised to 0
+  // call mj_forward, expect clock to report 0
+  mj_forward(model, data);
   EXPECT_EQ(data->sensordata[0], 0.0);
 
   // call mj_step, mj_step1, expect clock to be incremented by timestep