diff --git a/Xi/exevents.c b/Xi/exevents.c
index 29dceca0e..89adf4f30 100644
--- a/Xi/exevents.c
+++ b/Xi/exevents.c
@@ -891,6 +891,8 @@ ProcessOtherEvent(InternalEvent *ev, DeviceIntPtr device)
     DeviceIntPtr mouse = NULL, kbd = NULL;
     DeviceEvent *event = (DeviceEvent*)ev;
 
+    CHECKEVENT(ev);
+
     if (IsPointerDevice(device))
     {
         kbd = GetPairedDevice(device);
diff --git a/dix/events.c b/dix/events.c
index 5702d2f21..c21e57ebe 100644
--- a/dix/events.c
+++ b/dix/events.c
@@ -2247,6 +2247,8 @@ DeliverDeviceEvents(WindowPtr pWin, InternalEvent *event, GrabPtr grab,
     xEvent *xE = NULL;
     int rc, count = 0;
 
+    CHECKEVENT(event);
+
     /* XXX: In theory, we could pass the internal events through to everything
      * and only convert just before hitting the wire. We can't do that yet, so
      * DDE is the last stop for internal events. From here onwards, we deal
@@ -2527,6 +2529,8 @@ CheckMotion(DeviceEvent *ev, DeviceIntPtr pDev)
     WindowPtr prevSpriteWin;
     SpritePtr pSprite = pDev->spriteInfo->sprite;
 
+    CHECKEVENT(ev);
+
     prevSpriteWin = pSprite->win;
 
     if (ev && !syncEvents.playingEvents)
diff --git a/include/events.h b/include/events.h
index b09116741..7bc83a6d7 100644
--- a/include/events.h
+++ b/include/events.h
@@ -62,6 +62,10 @@ enum {
     ET_Internal = 0xFF /* First byte */
 } EventType;
 
+#define CHECKEVENT(ev) if (ev && ((InternalEvent*)(ev))->u.any.header != 0xFF) \
+                          FatalError("Wrong event type %d.\n", \
+                                     ((InternalEvent*)(ev))->u.any.header);
+
 /**
  * Used for ALL input device events internal in the server until
  * copied into the matching protocol event.
diff --git a/mi/mieq.c b/mi/mieq.c
index 084660263..efec55a63 100644
--- a/mi/mieq.c
+++ b/mi/mieq.c
@@ -162,6 +162,8 @@ mieqEnqueue(DeviceIntPtr pDev, InternalEvent *e)
     pthread_mutex_lock(&miEventQueueMutex);
 #endif
 
+    CHECKEVENT(e);
+
     /* avoid merging events from different devices */
     if (e->u.any.type == ET_Motion)
         isMotion = pDev->id;
@@ -285,6 +287,8 @@ static void
 FixUpEventForMaster(DeviceIntPtr mdev, DeviceIntPtr sdev,
                     InternalEvent* original, InternalEvent *master)
 {
+    CHECKEVENT(original);
+    CHECKEVENT(master);
     /* Ensure chained button mappings, i.e. that the detail field is the
      * value of the mapped button on the SD, not the physical button */
     if (original->u.any.type == ET_ButtonPress ||
@@ -312,6 +316,8 @@ CopyGetMasterEvent(DeviceIntPtr mdev, DeviceIntPtr sdev,
     int len = original->u.any.length;
     InternalEvent *mevent;
 
+    CHECKEVENT(original);
+
     if (master->evlen < len)
         SetMinimumEventSize(master, 1, len);
 
@@ -337,6 +343,8 @@ mieqProcessDeviceEvent(DeviceIntPtr dev,
     int x = 0, y = 0;
     DeviceIntPtr master;
 
+    CHECKEVENT(event);
+
     /* Custom event handler */
     handler = miEventQueue.handlers[event->u.any.type];