diff --git a/src/synaptics.c b/src/synaptics.c
index 77eb5be..d2fabeb 100644
--- a/src/synaptics.c
+++ b/src/synaptics.c
@@ -2996,6 +2996,8 @@ UpdateTouchState(InputInfoPtr pInfo, struct SynapticsHwState *hw)
 
     for (i = 0; i < hw->num_mt_mask; i++)
     {
+        assert(priv->num_active_touches >= 0);
+        assert(priv->num_active_touches <= priv->num_slots);
         if (hw->slot_state[i] == SLOTSTATE_OPEN)
         {
             priv->open_slots[priv->num_active_touches] = i;
@@ -3016,6 +3018,8 @@ UpdateTouchState(InputInfoPtr pInfo, struct SynapticsHwState *hw)
 
             priv->num_active_touches--;
         }
+        assert(priv->num_active_touches >= 0);
+        assert(priv->num_active_touches <= priv->num_slots);
     }
 
     SynapticsResetTouchHwState(hw, FALSE);