diff --git a/mod/forum/db/mysql.php b/mod/forum/db/mysql.php
index da719e10cd4..544cec6eeff 100644
--- a/mod/forum/db/mysql.php
+++ b/mod/forum/db/mysql.php
@@ -220,8 +220,8 @@ function forum_upgrade($oldversion) {
         include_once( "$CFG->dirroot/lib/wiki_to_markdown.php" );
         $wtm = new WikiToMarkdown();
         $sql = "select course from {$CFG->prefix}forum_discussions, {$CFG->prefix}forum_posts ";
-        $sql .=  "where forum_posts.discussions = forum_discussions.id ";
-        $sql .=  "and forum_posts.id = ";
+        $sql .=  "where {$CFG->prefix}forum_posts.discussions = {$CFG->prefix}forum_discussions.id ";
+        $sql .=  "and {$CFG->prefix}forum_posts.id = ";
         $wtm->update( 'forum_posts','message','format',$sql );
     }
 
diff --git a/mod/forum/db/postgres7.php b/mod/forum/db/postgres7.php
index 331b004d704..04fce786218 100644
--- a/mod/forum/db/postgres7.php
+++ b/mod/forum/db/postgres7.php
@@ -163,8 +163,8 @@ function forum_upgrade($oldversion) {
         include_once( "$CFG->dirroot/lib/wiki_to_markdown.php" );
         $wtm = new WikiToMarkdown();
         $sql = "select course from {$CFG->prefix}forum_discussions, {$CFG->prefix}forum_posts ";
-        $sql .=  "where forum_posts.discussions = forum_discussions.id ";
-        $sql .=  "and forum_posts.id = ";
+        $sql .=  "where {$CFG->prefix}forum_posts.discussions = {$CFG->prefix}forum_discussions.id ";
+        $sql .=  "and {$CFG->prefix}forum_posts.id = ";
         $wtm->update( 'forum_posts','message','format',$sql );
     }