diff --git a/docs/customizing.htm b/docs/customizing.htm
index fdefd0ad..f2cb98c3 100644
--- a/docs/customizing.htm
+++ b/docs/customizing.htm
@@ -7408,6 +7408,14 @@ growing_base = 50.0, degree_F</pre>
                 <td>ppm</td>
                 <td>ppm</td>
             </tr>
+            <tr>
+                <td class="first_col">group_frequency</td>
+                <td></td>
+                <td>hertz</td>
+                <td>hertz</td>
+                <td>hertz</td>
+                <td>hertz</td>
+            </tr>
             <tr>
                 <td class="first_col">group_illuminance</td>
                 <td>illuminance</td>