diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index b26f6f73f7..3e0c151415 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -3926,6 +3926,12 @@
 <!--                <para role="glossdeffirst"><imagedata fileref="figures/define-generic.png" /> -->
                     Additional GNU <filename>make</filename> options.
                 </para>
+
+                <para>
+                    Because the <filename>EXTRA_OEMAKE</filename> defaults to
+                    "", you need to set the variable to specify any required
+                    GNU options.
+                </para>
             </glossdef>
         </glossentry>