diff --git a/lib/classes/plugininfo/qbehaviour.php b/lib/classes/plugininfo/qbehaviour.php
index b33e22601a0..85de13462a8 100644
--- a/lib/classes/plugininfo/qbehaviour.php
+++ b/lib/classes/plugininfo/qbehaviour.php
@@ -60,6 +60,11 @@ class qbehaviour extends base {
     public function is_uninstall_allowed() {
         global $DB;
 
+        if ($this->name === 'missing') {
+            // qbehaviour_missing is used by the system. It cannot be uninstalled.
+            return false;
+        }
+
         return !$DB->record_exists('question_attempts', array('behaviour' => $this->name));
     }
 
diff --git a/lib/classes/plugininfo/qtype.php b/lib/classes/plugininfo/qtype.php
index 375ee1a10f0..deeae712d8b 100644
--- a/lib/classes/plugininfo/qtype.php
+++ b/lib/classes/plugininfo/qtype.php
@@ -68,6 +68,11 @@ class qtype extends base {
     public function is_uninstall_allowed() {
         global $DB;
 
+        if ($this->name === 'missingtype') {
+            // qtype_missingtype is used by the system. It cannot be uninstalled.
+            return false;
+        }
+
         return !$DB->record_exists('question', array('qtype' => $this->name));
     }