diff --git a/reactos/lib/sdk/crt/mbstring/mbslwr.c b/reactos/lib/sdk/crt/mbstring/mbslwr.c
index d2ad5bcc3e2..10d5e11296c 100644
--- a/reactos/lib/sdk/crt/mbstring/mbslwr.c
+++ b/reactos/lib/sdk/crt/mbstring/mbslwr.c
@@ -1,12 +1,10 @@
 /*
  * COPYRIGHT:   See COPYING in the top level directory
  * PROJECT:     ReactOS system libraries
- * FILE:        lib/sdk/crt/mbstring/ismbpun.c
- * PURPOSE:
- * PROGRAMER:   
- * UPDATE HISTORY:
- *              05/30/08: Samuel Serapion adapted from PROJECT C Library
- *
+ * FILE:        lib/sdk/crt/mbstring/mbslwr.c
+ * PURPOSE:     Multibyte lowercase functions
+ * PROGRAMER:   Eric Kohl
+ *              Samuel Serapion, adapted from PROJECT C Library
  */
 
 #include <precomp.h>
@@ -33,14 +31,24 @@ unsigned int _mbctolower(unsigned int c)
  */
 unsigned char * _mbslwr(unsigned char *x)
 {
-    unsigned char  *y=x;
+    unsigned char *y=x;
 
-    while (*y) {
-        if (!_ismbblead(*y)) {
+    if (x == NULL)
+    {
+        return NULL;
+    }
+
+    while (*y) 
+    {
+        if (!_ismbblead(*y)) 
+        {
             *y = tolower(*y);
-	    } else {
-	        *y=_mbctolower(*(unsigned short *)y);
-	        y++;
+            y++;
+        } 
+        else
+        {
+            *y = _mbctolower(*(unsigned short *)y);
+            y++;
         }
     }
     return x;