diff --git a/functions/mysql.xml b/functions/mysql.xml
index 47c37bc30e..0f3281282c 100644
--- a/functions/mysql.xml
+++ b/functions/mysql.xml
@@ -61,6 +61,9 @@
      statements which modify records. To retrieve the number of rows
      returned from a SELECT, use <function>mysql_num_rows</function>.
     </para>
+    <para>
+     If the last query failed, this function will return -1.
+    </para>
    </refsect1>
   </refentry>