Index: dll/win32/uxtheme/msstyles.c
===================================================================
--- dll/win32/uxtheme/msstyles.c	(revision 56069)
+++ dll/win32/uxtheme/msstyles.c	(working copy)
@@ -1000,11 +1000,12 @@
         lstrcpynW(szClassName, start, sizeof(szClassName)/sizeof(szClassName[0]));
         cls = MSSTYLES_FindClass(tfActiveTheme, pszAppName, szClassName);
     }
+	/*Set the Error in case pszClassList doesn't contain a valid Class */
     if(cls) {
         TRACE("Opened app %s, class %s from list %s\n", debugstr_w(cls->szAppName), debugstr_w(cls->szClassName), debugstr_w(pszClassList));
 	cls->tf = tfActiveTheme;
 	cls->tf->dwRefCount++;
-    }
+    } else SetLastError(E_PROP_ID_UNSUPPORTED);
     return cls;
 }
 
Index: dll/win32/uxtheme/system.c
===================================================================
--- dll/win32/uxtheme/system.c	(revision 56069)
+++ dll/win32/uxtheme/system.c	(working copy)
@@ -633,7 +633,7 @@
     
     if(pszClassList == NULL)
 		SetLastError(E_POINTER);
-
+	
     if(bThemeActive)
     {
         pszAppName = UXTHEME_GetWindowProperty(hwnd, atSubAppName, szAppBuff, sizeof(szAppBuff)/sizeof(szAppBuff[0]));
@@ -645,8 +645,10 @@
         if (pszUseClassList)
             hTheme = MSSTYLES_OpenThemeClass(pszAppName, pszUseClassList);
     }
+	/*In case hwnd is not valid, set the Error */
     if(IsWindow(hwnd))
         SetPropW(hwnd, (LPCWSTR)MAKEINTATOM(atWindowTheme), hTheme);
+	else SetLastError(E_PROP_ID_UNSUPPORTED);
     TRACE(" = %p\n", hTheme);
 
     return hTheme;