diff --git a/code/userprog/ksyscall.h b/code/userprog/ksyscall.h
index 68718f1..afe38e8 100755
--- a/code/userprog/ksyscall.h
+++ b/code/userprog/ksyscall.h
@@ -53,6 +53,8 @@ OpenFileId SysOpen(char *name) {
 }
 
 int SysWrite(char *buffer, int size, OpenFileId id) {
+  if (size < 0)  // Invalid size
+    return -1;
   if (id == 1) {  // Console output (stdout)
     for (int i = 0; i < size; i++)
       kernel->synchConsoleOut->PutChar(buffer[i]);
@@ -65,6 +67,8 @@ int SysWrite(char *buffer, int size, OpenFileId id) {
 }
 
 int SysRead(char *buffer, int size, OpenFileId id) {
+  if (size < 0)  // Invalid size
+    return -1;
   if (id == 0) {  // Console input (stdin)
     for (int i = 0; i < size; i++)
       buffer[i] = kernel->synchConsoleIn->GetChar();