diff --git a/reference/filter/functions/filter-var.xml b/reference/filter/functions/filter-var.xml
index 392ead1eb7..43dc0d6310 100644
--- a/reference/filter/functions/filter-var.xml
+++ b/reference/filter/functions/filter-var.xml
@@ -24,7 +24,9 @@
      <term><parameter>variable</parameter></term>
      <listitem>
       <para>
-       Value to filter.
+       Value to filter. Note that scalar values are <link
+       linkend="language.types.string.casting">converted to
+       string</link> internally before they are filtered.
       </para>
      </listitem>
     </varlistentry>