diff --git a/reference/classobj/functions/is-subclass-of.xml b/reference/classobj/functions/is-subclass-of.xml
index 55011dfe89..945d06b059 100644
--- a/reference/classobj/functions/is-subclass-of.xml
+++ b/reference/classobj/functions/is-subclass-of.xml
@@ -42,7 +42,8 @@
      <term><parameter>allow_string</parameter></term>
      <listitem>
       <para>
-       Whether to call autoloader if the class doesn't exist.
+       If this parameter set to false, string class name as <parameter>object</parameter>
+       is not allowed. This also prevents from calling autoloader if the class doesn't exist.
       </para>
      </listitem>
     </varlistentry>