diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index b829ae2384f..09192bc1c76 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -2964,7 +2964,7 @@ JS_SetPrivate(JSContext *cx, JSObject *obj, void *data)
 JS_PUBLIC_API(void *)
 JS_GetInstancePrivate(JSContext *cx, JSObject *obj, JSClass *clasp, jsval *argv)
 {
-    if (JS_InstanceOf(cx, obj, clasp, argv))
+    if (!JS_InstanceOf(cx, obj, clasp, argv))
         return NULL;
     return obj->getPrivate();
 }
diff --git a/js/src/jsxml.cpp b/js/src/jsxml.cpp
index 6305b06efb8..02ef6874855 100644
--- a/js/src/jsxml.cpp
+++ b/js/src/jsxml.cpp
@@ -7528,6 +7528,7 @@ GetPrivate(JSContext *cx, JSObject *obj, const char *method)
         JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
                              JSMSG_INCOMPATIBLE_METHOD,
                              js_XML_str, method, obj->getClass()->name);
+        return NULL;
     }
     return (JSXML *)obj->getPrivate();
 }