diff --git a/reactos/lib/user32/windows/class.c b/reactos/lib/user32/windows/class.c
index b54630faaa1..07e72425591 100644
--- a/reactos/lib/user32/windows/class.c
+++ b/reactos/lib/user32/windows/class.c
@@ -1,4 +1,4 @@
-/* $Id: class.c,v 1.32 2003/08/19 00:49:42 weiden Exp $
+/* $Id: class.c,v 1.33 2003/08/19 01:03:41 weiden Exp $
  *
  * COPYRIGHT:       See COPYING in the top level directory
  * PROJECT:         ReactOS user32.dll
@@ -137,6 +137,13 @@ GetClassInfoA(
 {
 	WNDCLASSEXA w;
 	WINBOOL retval;
+	
+	if(!lpClassName || !lpWndClass)
+	{
+		SetLastError(ERROR_INVALID_PARAMETER);
+		return FALSE;
+	}
+	
 	retval = GetClassInfoExA(hInstance,lpClassName,&w);
     RtlCopyMemory (lpWndClass,&w.style,sizeof(WNDCLASSA));
 	return retval;
@@ -154,6 +161,13 @@ GetClassInfoW(
 {
 	WNDCLASSEXW w;
 	WINBOOL retval;
+	
+	if(!lpClassName || !lpWndClass)
+	{
+		SetLastError(ERROR_INVALID_PARAMETER);
+		return FALSE;
+	}
+	
 	retval = GetClassInfoExW(hInstance,lpClassName,&w);
     RtlCopyMemory (lpWndClass,&w.style,sizeof(WNDCLASSW));
 	return retval;
@@ -221,6 +235,9 @@ GetClassNameA(
   int result;
   LPWSTR ClassNameW;
   NTSTATUS Status;
+  
+  if(!lpClassName)
+    return 0;
 
   ClassNameW = HEAP_alloc ( (nMaxCount+1)*sizeof(WCHAR) );
 
@@ -249,6 +266,9 @@ GetClassNameW(
 {
   int result;
   LPWSTR ClassNameW;
+  
+  if(!lpClassName)
+    return 0;
 
   ClassNameW = HEAP_alloc ( (nMaxCount+1) * sizeof(WCHAR) );
 
diff --git a/reactos/subsys/win32k/ntuser/class.c b/reactos/subsys/win32k/ntuser/class.c
index 1df6da48f8d..172be21429a 100644
--- a/reactos/subsys/win32k/ntuser/class.c
+++ b/reactos/subsys/win32k/ntuser/class.c
@@ -16,7 +16,7 @@
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
-/* $Id: class.c,v 1.32 2003/08/19 00:36:40 weiden Exp $
+/* $Id: class.c,v 1.33 2003/08/19 01:03:41 weiden Exp $
  *
  * COPYRIGHT:        See COPYING in the top level directory
  * PROJECT:          ReactOS kernel
@@ -257,9 +257,10 @@ NtUserGetClassName (
 
   WindowObject = W32kGetWindowObject(hWnd);
   if (WindowObject == NULL)
-    {
-      return(0);
-    }
+  {
+    SetLastWin32Error(ERROR_INVALID_WINDOW_HANDLE);
+    return 0;
+  }
   Ret = W32kGetClassName(WindowObject, lpClassName, nMaxCount);
   W32kReleaseWindowObject(WindowObject);
   return(Ret);
@@ -450,9 +451,10 @@ NtUserGetClassLong(HWND hWnd, DWORD Offset, BOOL Ansi)
 
   WindowObject = W32kGetWindowObject(hWnd);
   if (WindowObject == NULL)
-    {
-      return(0);
-    }
+  {
+    SetLastWin32Error(ERROR_INVALID_WINDOW_HANDLE);
+    return 0;
+  }
   Ret = W32kGetClassLong(WindowObject, Offset, Ansi);
   W32kReleaseWindowObject(WindowObject);
   return(Ret);
@@ -528,9 +530,10 @@ NtUserSetClassLong(HWND hWnd,
 
   WindowObject = W32kGetWindowObject(hWnd);
   if (WindowObject == NULL)
-    {
-      return(0);
-    }
+  {
+    SetLastWin32Error(ERROR_INVALID_WINDOW_HANDLE);
+    return 0;
+  }
   Ret = W32kGetClassLong(WindowObject, Offset, Ansi);
   W32kSetClassLong(WindowObject, Offset, dwNewLong, Ansi);
   W32kReleaseWindowObject(WindowObject);