diff --git a/doc/manpages/dinit-service.5.m4 b/doc/manpages/dinit-service.5.m4
index 11775fdd..29fa1459 100644
--- a/doc/manpages/dinit-service.5.m4
+++ b/doc/manpages/dinit-service.5.m4
@@ -163,9 +163,11 @@ The value is subject to variable substitution (see \fBVARIABLE SUBSTITUTION\fR).
 Specifies which user to run the process(es) for this service as.
 Specify as a username or numeric ID.
 If specified by name, the group for the process will also be set to the primary
-group of the specified user.
-Supplementary groups will be initialized unless disabled, not supported on
-the platform, or the user could not be found in passwd database.
+group of the specified user, and supplementary groups will be initialised (unless support
+for them is disabled) according to the system's group database.
+If specified by number, the group for the process will remain the same as that of the
+running \fBdinit\fR process, and all supplementary groups will be dropped (unless support
+has been disabled).
 .TP
 \fBenv\-file\fR = \fIfile\fR
 Specifies a file containing value assignments for environment variables, in the same
diff --git a/src/includes/load-service.h b/src/includes/load-service.h
index 6d6973b1..61f4801b 100644
--- a/src/includes/load-service.h
+++ b/src/includes/load-service.h
@@ -1014,7 +1014,13 @@ class service_settings_wrapper
         // likewise for "run as" gid/uid, but only if we aren't supporting supplementary group initialisation
         // (if we do support supplementary groups, run_as_gid==-1 means "use the user groups including
         // supplementary groups" whereas run_as_gid==X means "use group X with no supplementary groups").
-#if !USE_INITGROUPS
+#if USE_INITGROUPS
+        if (run_as_uid_gid == -1 && run_as_gid == -1) {
+            // run-as-UID specified by number; set run_as_gid to inhibit initialisation of
+            // supplementary groups
+            run_as_gid = getgid();
+        }
+#else // !USE_INITGROUPS
         if (run_as_gid == (gid_t)-1) run_as_gid = run_as_uid_gid;
 #endif