diff --git a/reference/filter/filters.xml b/reference/filter/filters.xml
index 32548f7ea9..0c6ee2fca5 100644
--- a/reference/filter/filters.xml
+++ b/reference/filter/filters.xml
@@ -105,7 +105,7 @@
          <constant>FILTER_FLAG_PATH_REQUIRED</constant>,
          <constant>FILTER_FLAG_QUERY_REQUIRED</constant>
         </entry>
-        <entry>Validates value as URL (according to <link xlink:href="&url.rfc;2396">&url.rfc;2396</link>), optionally with required components. Note that the function will only find ASCII URLs to be valid; internationalized domain names (containing non-ASCII characters) will fail.</entry>
+        <entry>Validates value as URL (according to <link xlink:href="&url.rfc;2396">&url.rfc;2396</link>), optionally with required components. Beware a valid URL may not specify the HTTP protocol <parameter>http://</parameter> so further validation may be required to determine the URL uses an expected protocol, e.g. <parameter>ssh://</parameter> or <parameter>mailto:</parameter>. Note that the function will only find ASCII URLs to be valid; internationalized domain names (containing non-ASCII characters) will fail.</entry>
        </row>
       </tbody>
      </tgroup>