diff --git a/win32ss/user/ntuser/class.c b/win32ss/user/ntuser/class.c
index 83fe9009b34..d669340baf7 100644
--- a/win32ss/user/ntuser/class.c
+++ b/win32ss/user/ntuser/class.c
@@ -2815,6 +2815,12 @@ NtUserGetClassName (IN HWND hWnd,
         {
             ProbeForWriteUnicodeString(ClassName);
             CapturedClassName = *ClassName;
+            if (CapturedClassName.Length != 0)
+            {
+                ProbeForRead(CapturedClassName.Buffer,
+                             CapturedClassName.Length,
+                             sizeof(WCHAR));
+            }
 
             /* Get the class name */
             Ret = UserGetClassName(Window->pcls,