diff --git a/man/systemctl.xml b/man/systemctl.xml
index aac17ae01a..ed0cafff8b 100644
--- a/man/systemctl.xml
+++ b/man/systemctl.xml
@@ -616,6 +616,10 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
                     <entry><literal>all</literal></entry>
                     <entry>All of the above</entry>
                   </row>
+                  <row>
+                    <entry><literal>help</literal></entry>
+                    <entry>Show the supported values and exit</entry>
+                  </row>
                 </tbody>
               </tgroup>
             </table>
diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index fdbde7dc5e..55fa386e7a 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -958,7 +958,7 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
 
                 case ARG_WHAT:
                         if (isempty(optarg))
-                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "--what= requires arguments.");
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "--what= requires arguments (see --what=help).");
 
                         for (const char *p = optarg;;) {
                                 _cleanup_free_ char *k = NULL;