diff --git a/sql/handler.cc b/sql/handler.cc
index f966a9099ee..ac959cb62f2 100644
--- a/sql/handler.cc
+++ b/sql/handler.cc
@@ -413,7 +413,13 @@ int ha_finalize_handlerton(st_plugin_int *plugin)
     reuse an array slot. Otherwise the number of uninstall/install
     cycles would be limited.
   */
-  hton2plugin[hton->slot]= NULL;
+  if (hton->slot != HA_SLOT_UNDEF)
+  {
+    /* Make sure we are not unpluging another plugin */
+    DBUG_ASSERT(hton2plugin[hton->slot] == plugin);
+    DBUG_ASSERT(hton->slot < MAX_HA);
+    hton2plugin[hton->slot]= NULL;
+  }
 
   my_free((uchar*)hton, MYF(0));
 
@@ -438,6 +444,7 @@ int ha_initialize_handlerton(st_plugin_int *plugin)
     goto err_no_hton_memory;
   }
 
+  hton->slot= HA_SLOT_UNDEF;
   /* Historical Requirement */
   plugin->data= hton; // shortcut for the future
   if (plugin->plugin->init && plugin->plugin->init(hton))
diff --git a/sql/handler.h b/sql/handler.h
index a281aaa0ad7..ea0b134e53d 100644
--- a/sql/handler.h
+++ b/sql/handler.h
@@ -216,6 +216,13 @@
 */
 #define MAX_HA 15
 
+/*
+  Use this instead of 0 as the initial value for the slot number of
+  handlerton, so that we can distinguish uninitialized slot number
+  from slot 0.
+*/
+#define HA_SLOT_UNDEF ((uint)-1)
+
 /*
   Parameters for open() (in register form->filestat)
   HA_GET_INFO does an implicit HA_ABORT_IF_LOCKED