--- 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));
--- 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;
 }

--- 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));