--- a/content/html/content/src/nsHTMLCanvasElement.cpp
+++ b/content/html/content/src/nsHTMLCanvasElement.cpp
@@ -325,17 +325,17 @@ nsHTMLCanvasElement::ToDataURL(nsAString
       return NS_ERROR_DOM_SYNTAX_ERR;
     JSString *type = JS_ValueToString(ctx, argv[0]);
     return ToDataURLImpl (nsDependentString(reinterpret_cast<PRUnichar*>((JS_GetStringChars(type)))),
                           EmptyString(), aDataURL);
   }
 
   // 2-arg case; trusted only (checked above), convert to mime type with params
   if (argc == 2) {
-    if (!JSVAL_IS_STRING(argv[0]) && !JSVAL_IS_STRING(argv[1]))
+    if (!JSVAL_IS_STRING(argv[0]) || !JSVAL_IS_STRING(argv[1]))
       return NS_ERROR_DOM_SYNTAX_ERR;
 
     JSString *type, *params;
     type = JS_ValueToString(ctx, argv[0]);
     params = JS_ValueToString(ctx, argv[1]);
 
     return ToDataURLImpl (nsDependentString(reinterpret_cast<PRUnichar*>(JS_GetStringChars(type))),
                           nsDependentString(reinterpret_cast<PRUnichar*>(JS_GetStringChars(params))),