diff --git a/man/systemd.generator.xml b/man/systemd.generator.xml
index 1f916ac65e..d837afb6f9 100644
--- a/man/systemd.generator.xml
+++ b/man/systemd.generator.xml
@@ -26,8 +26,8 @@
     <cmdsynopsis>
       <command index='false'>/path/to/generator</command>
       <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
-      <arg choice="plain"><replaceable>early-dir</replaceable></arg>
-      <arg choice="plain"><replaceable>late-dir</replaceable></arg>
+      <arg choice="option"><replaceable>early-dir</replaceable></arg>
+      <arg choice="option"><replaceable>late-dir</replaceable></arg>
     </cmdsynopsis>
 
     <para>
@@ -56,13 +56,13 @@
     that they can extend the unit file hierarchy the service manager subsequently loads and operates
     on.</para>
 
-    <para>Each generator is called with three directory paths that are to be used for generator output. In
-    these three directories, generators may dynamically generate unit files (regular ones, instances, as well
-    as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic links to unit files to
-    add additional dependencies, create aliases, or instantiate existing templates. Those directories are
-    included in the unit load path of
-    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, allowing
-    generated configuration to extend or override existing definitions.</para>
+    <para><command>systemd</command> will call each generator with three directory paths that are to be used
+    for generator output. In these three directories, generators may dynamically generate unit files (regular
+    ones, instances, as well as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic
+    links to unit files to add additional dependencies, create aliases, or instantiate existing templates.
+    Those directories are included in the unit load path, allowing generated configuration to extend or
+    override existing definitions. For tests, generators may be called with just one argument; the generator
+    should assume that all three paths are the same in that case.</para>
 
     <para>Directory paths for generator output differ by priority: <filename>…/generator.early</filename> has
     priority higher than the admin configuration in <filename>/etc/</filename>, while
@@ -96,7 +96,8 @@
     <para>Generators are invoked with three arguments: paths to directories where generators can place their
     generated unit files or symlinks. By default those paths are runtime directories that are included in the
     search path of <command>systemd</command>, but a generator may be called with different paths for
-    debugging purposes.</para>
+    debugging purposes. If only one argument is provided, the generator should use the same directory as the
+    the three output paths.</para>
 
     <orderedlist>
       <listitem>
diff --git a/src/shared/generator.h b/src/shared/generator.h
index bd768daf49..1b4f36ac53 100644
--- a/src/shared/generator.h
+++ b/src/shared/generator.h
@@ -3,6 +3,7 @@
 
 #include <stdio.h>
 
+#include "macro.h"
 #include "main-func.h"
 
 int generator_open_unit_file(
@@ -86,11 +87,11 @@ void log_setup_generator(void);
         _DEFINE_MAIN_FUNCTION(                                          \
                 ({                                                      \
                         log_setup_generator();                          \
-                        if (argc > 1 && argc != 4)                      \
+                        if (!IN_SET(argc, 2, 4))                        \
                                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), \
-                                                       "This program takes zero or three arguments."); \
+                                                       "This program takes one or three arguments."); \
                 }),                                                     \
-                impl(argc > 1 ? argv[1] : "/tmp",                       \
-                     argc > 1 ? argv[2] : "/tmp",                       \
-                     argc > 1 ? argv[3] : "/tmp"),                      \
+                impl(argv[1],                                           \
+                     argv[argc == 4 ? 2 : 1],                           \
+                     argv[argc == 4 ? 3 : 1]),                          \
                 r < 0 ? EXIT_FAILURE : EXIT_SUCCESS)