window.Document doesn't exist in IE8

js/tooltip.js

@@ -52,7 +52,7 @@
     this.options   = this.getOptions(options)
     this.$viewport = this.options.viewport && $(this.options.viewport.selector || this.options.viewport)
 
-    if (this.$element[0] instanceof window.Document && !this.options.selector) {
+    if (this.$element[0] instanceof document.constructor && !this.options.selector) {
       throw new Error('`selector` option must be specified when initializing ' + this.type + ' on the window.document object!');
     }