diff --git a/libc/inc/stddef.h b/libc/inc/stddef.h
index 6eb5b2f..f22e546 100644
--- a/libc/inc/stddef.h
+++ b/libc/inc/stddef.h
@@ -37,10 +37,10 @@ typedef unsigned int ino_t;
 typedef unsigned int dev_t;
 
 /// The type of user-id.
-typedef unsigned int uid_t;
+typedef int uid_t;
 
 /// The type of group-id.
-typedef unsigned int gid_t;
+typedef int gid_t;
 
 /// The type of offset.
 typedef long int off_t;
diff --git a/mentos/src/process/scheduler.c b/mentos/src/process/scheduler.c
index e0192f1..8d1c7a1 100644
--- a/mentos/src/process/scheduler.c
+++ b/mentos/src/process/scheduler.c
@@ -386,6 +386,9 @@ uid_t sys_getuid(void)
 
 int sys_setuid(uid_t uid)
 {
+    if (uid < 0) {
+        return -EINVAL;
+    }
     if (runqueue.curr && (runqueue.curr->uid == 0)) {
         runqueue.curr->uid = uid;
         return 0;
@@ -403,6 +406,9 @@ pid_t sys_getgid(void)
 
 int sys_setgid(pid_t gid)
 {
+    if (gid < 0) {
+        return -EINVAL;
+    }
     if (runqueue.curr && (runqueue.curr->uid == 0)) {
         runqueue.curr->gid = gid;
         return 0;