--- /dev/null
+diff --git a/src/polkit/polkitunixgroup.c b/src/polkit/polkitunixgroup.c
+index c57a1aaacbb13c4e4297dd812cf5904f2f427b03..309f68918895e0f8b547f8c06f89c6fb1326fe20 100644
+--- a/src/polkit/polkitunixgroup.c
++++ b/src/polkit/polkitunixgroup.c
+@@ -71,6 +71,7 @@ G_DEFINE_TYPE_WITH_CODE (PolkitUnixGroup, polkit_unix_group, G_TYPE_OBJECT,
+ static void
+ polkit_unix_group_init (PolkitUnixGroup *unix_group)
+ {
++ unix_group->gid = -1; /* (git_t) -1 is not a valid GID under Linux */
+ }
+
+ static void
+@@ -100,11 +101,14 @@ polkit_unix_group_set_property (GObject *object,
+ GParamSpec *pspec)
+ {
+ PolkitUnixGroup *unix_group = POLKIT_UNIX_GROUP (object);
++ gint val;
+
+ switch (prop_id)
+ {
+ case PROP_GID:
+- unix_group->gid = g_value_get_int (value);
++ val = g_value_get_int (value);
++ g_return_if_fail (val != -1);
++ unix_group->gid = val;
+ break;
+
+ default:
+@@ -131,9 +135,9 @@ polkit_unix_group_class_init (PolkitUnixGroupClass *klass)
+ g_param_spec_int ("gid",
+ "Group ID",
+ "The UNIX group ID",
+- 0,
++ G_MININT,
+ G_MAXINT,
+- 0,
++ -1,
+ G_PARAM_CONSTRUCT |
+ G_PARAM_READWRITE |
+ G_PARAM_STATIC_NAME |
+@@ -166,9 +170,10 @@ polkit_unix_group_get_gid (PolkitUnixGroup *group)
+ */
+ void
+ polkit_unix_group_set_gid (PolkitUnixGroup *group,
+- gint gid)
++ gint gid)
+ {
+ g_return_if_fail (POLKIT_IS_UNIX_GROUP (group));
++ g_return_if_fail (gid != -1);
+ group->gid = gid;
+ }
+
+@@ -183,6 +188,8 @@ polkit_unix_group_set_gid (PolkitUnixGroup *group,
+ PolkitIdentity *
+ polkit_unix_group_new (gint gid)
+ {
++ g_return_val_if_fail (gid != -1, NULL);
++
+ return POLKIT_IDENTITY (g_object_new (POLKIT_TYPE_UNIX_GROUP,
+ "gid", gid,
+ NULL));
+diff --git a/src/polkit/polkitunixprocess.c b/src/polkit/polkitunixprocess.c
+index 972b7776825d5ccf677ed12ed620fc0c52352547..b02b25894ad120d88ea21d4c96ac8dca1821fcf2 100644
+--- a/src/polkit/polkitunixprocess.c
++++ b/src/polkit/polkitunixprocess.c
+@@ -159,9 +159,14 @@ polkit_unix_process_set_property (GObject *object,
+ polkit_unix_process_set_pid (unix_process, g_value_get_int (value));
+ break;
+
+- case PROP_UID:
+- polkit_unix_process_set_uid (unix_process, g_value_get_int (value));
++ case PROP_UID: {
++ gint val;
++
++ val = g_value_get_int (value);
++ g_return_if_fail (val != -1);
++ polkit_unix_process_set_uid (unix_process, val);
+ break;
++ }
+
+ case PROP_START_TIME:
+ polkit_unix_process_set_start_time (unix_process, g_value_get_uint64 (value));
+@@ -239,7 +244,7 @@ polkit_unix_process_class_init (PolkitUnixProcessClass *klass)
+ g_param_spec_int ("uid",
+ "User ID",
+ "The UNIX user ID",
+- -1,
++ G_MININT,
+ G_MAXINT,
+ -1,
+ G_PARAM_CONSTRUCT |
+@@ -303,7 +308,6 @@ polkit_unix_process_set_uid (PolkitUnixProcess *process,
+ gint uid)
+ {
+ g_return_if_fail (POLKIT_IS_UNIX_PROCESS (process));
+- g_return_if_fail (uid >= -1);
+ process->uid = uid;
+ }
+
+diff --git a/src/polkit/polkitunixuser.c b/src/polkit/polkitunixuser.c
+index 8bfd3a1fb05ddb56adebd097569a9977b7b922f3..234a6976c573ac65200ee08228cd50111f0c769b 100644
+--- a/src/polkit/polkitunixuser.c
++++ b/src/polkit/polkitunixuser.c
+@@ -72,6 +72,7 @@ G_DEFINE_TYPE_WITH_CODE (PolkitUnixUser, polkit_unix_user, G_TYPE_OBJECT,
+ static void
+ polkit_unix_user_init (PolkitUnixUser *unix_user)
+ {
++ unix_user->uid = -1; /* (uid_t) -1 is not a valid UID under Linux */
+ unix_user->name = NULL;
+ }
+
+@@ -112,11 +113,14 @@ polkit_unix_user_set_property (GObject *object,
+ GParamSpec *pspec)
+ {
+ PolkitUnixUser *unix_user = POLKIT_UNIX_USER (object);
++ gint val;
+
+ switch (prop_id)
+ {
+ case PROP_UID:
+- unix_user->uid = g_value_get_int (value);
++ val = g_value_get_int (value);
++ g_return_if_fail (val != -1);
++ unix_user->uid = val;
+ break;
+
+ default:
+@@ -144,9 +148,9 @@ polkit_unix_user_class_init (PolkitUnixUserClass *klass)
+ g_param_spec_int ("uid",
+ "User ID",
+ "The UNIX user ID",
+- 0,
++ G_MININT,
+ G_MAXINT,
+- 0,
++ -1,
+ G_PARAM_CONSTRUCT |
+ G_PARAM_READWRITE |
+ G_PARAM_STATIC_NAME |
+@@ -182,6 +186,7 @@ polkit_unix_user_set_uid (PolkitUnixUser *user,
+ gint uid)
+ {
+ g_return_if_fail (POLKIT_IS_UNIX_USER (user));
++ g_return_if_fail (uid != -1);
+ user->uid = uid;
+ }
+
+@@ -196,6 +201,8 @@ polkit_unix_user_set_uid (PolkitUnixUser *user,
+ PolkitIdentity *
+ polkit_unix_user_new (gint uid)
+ {
++ g_return_val_if_fail (uid != -1, NULL);
++
+ return POLKIT_IDENTITY (g_object_new (POLKIT_TYPE_UNIX_USER,
+ "uid", uid,
+ NULL));
+diff --git a/test/data/etc/group b/test/data/etc/group
+index 12ef328b21b346ee3828ce3aaf15cca83858bd1d..b9acab97211fdf7db521dc0939b2dcfc2c9e350b 100644
+--- a/test/data/etc/group
++++ b/test/data/etc/group
+@@ -5,3 +5,4 @@ john:x:500:
+ jane:x:501:
+ sally:x:502:
+ henry:x:503:
++highuid2:x:4000000000:
+diff --git a/test/data/etc/passwd b/test/data/etc/passwd
+index 8544febcd8b1720e5577dfb3f0672a6fef29e701..5cf14a5620259f79806192ca935fee84a29ac96d 100644
+--- a/test/data/etc/passwd
++++ b/test/data/etc/passwd
+@@ -3,3 +3,5 @@ john:x:500:500:John Done:/home/john:/bin/bash
+ jane:x:501:501:Jane Smith:/home/jane:/bin/bash
+ sally:x:502:502:Sally Derp:/home/sally:/bin/bash
+ henry:x:503:503:Henry Herp:/home/henry:/bin/bash
++highuid1:x:2147483648:2147483648:The first high uid:/home/highuid1:/sbin/nologin
++highuid2:x:4000000000:4000000000:An example high uid:/home/example:/sbin/nologin
+diff --git a/test/data/etc/polkit-1/rules.d/10-testing.rules b/test/data/etc/polkit-1/rules.d/10-testing.rules
+index 446e62291b7fe4c5bacdceb1045350af1a9dc245..98bf062a08cb11fddb7df95d0bcdec1b1ac3587d 100644
+--- a/test/data/etc/polkit-1/rules.d/10-testing.rules
++++ b/test/data/etc/polkit-1/rules.d/10-testing.rules
+@@ -53,6 +53,27 @@ polkit.addRule(function(action, subject) {
+ }
+ });
+
++polkit.addRule(function(action, subject) {
++ if (action.id == "net.company.john_action") {
++ if (subject.user == "john") {
++ return polkit.Result.YES;
++ } else {
++ return polkit.Result.NO;
++ }
++ }
++});
++
++polkit.addRule(function(action, subject) {
++ if (action.id == "net.company.highuid2_action") {
++ if (subject.user == "highuid2") {
++ return polkit.Result.YES;
++ } else {
++ return polkit.Result.NO;
++ }
++ }
++});
++
++
+ // ---------------------------------------------------------------------
+ // variables
+
+diff --git a/test/polkitbackend/test-polkitbackendjsauthority.c b/test/polkitbackend/test-polkitbackendjsauthority.c
+index b484a26600dbde074ee7d8491f88624fdc83c39c..71aad23e2f5d1a7b15e138f23e6581a31498bad6 100644
+--- a/test/polkitbackend/test-polkitbackendjsauthority.c
++++ b/test/polkitbackend/test-polkitbackendjsauthority.c
+@@ -330,6 +330,78 @@ static const RulesTestCase rules_test_cases[] = {
+ NULL,
+ POLKIT_IMPLICIT_AUTHORIZATION_AUTHORIZED,
+ },
++
++ {
++ /* highuid1 is not a member of group 'users', see test/data/etc/group */
++ "group_membership_with_non_member(highuid22)",
++ "net.company.group.only_group_users",
++ "unix-user:highuid2",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
++
++ {
++ /* highuid2 is not a member of group 'users', see test/data/etc/group */
++ "group_membership_with_non_member(highuid21)",
++ "net.company.group.only_group_users",
++ "unix-user:highuid2",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
++
++ {
++ /* highuid1 is not a member of group 'users', see test/data/etc/group */
++ "group_membership_with_non_member(highuid24)",
++ "net.company.group.only_group_users",
++ "unix-user:2147483648",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
++
++ {
++ /* highuid2 is not a member of group 'users', see test/data/etc/group */
++ "group_membership_with_non_member(highuid23)",
++ "net.company.group.only_group_users",
++ "unix-user:4000000000",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
++
++ {
++ /* john is authorized to do this, see 10-testing.rules */
++ "john_action",
++ "net.company.john_action",
++ "unix-user:john",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_AUTHORIZED,
++ },
++
++ {
++ /* only john is authorized to do this, see 10-testing.rules */
++ "jane_action",
++ "net.company.john_action",
++ "unix-user:jane",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
++
++ {
++ /* highuid2 is authorized to do this, see 10-testing.rules */
++ "highuid2_action",
++ "net.company.highuid2_action",
++ "unix-user:highuid2",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_AUTHORIZED,
++ },
++
++ {
++ /* only highuid2 is authorized to do this, see 10-testing.rules */
++ "highuid1_action",
++ "net.company.highuid2_action",
++ "unix-user:highuid1",
++ NULL,
++ POLKIT_IMPLICIT_AUTHORIZATION_NOT_AUTHORIZED,
++ },
+ };
+
+ /* ---------------------------------------------------------------------------------------------------- */
+