diff --git a/man/sd-id128.xml b/man/sd-id128.xml
index b5a7e77f8c..4d2f6eed1f 100644
--- a/man/sd-id128.xml
+++ b/man/sd-id128.xml
@@ -85,7 +85,7 @@
 
     <programlisting>#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)</programlisting>
 
-    <para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
+    <para><constant>SD_ID128_NULL</constant> may be used to refer to the 128bit ID consisting of only NUL
     bytes.</para>
 
     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a