diff --git a/canvasblocker.xpi b/canvasblocker.xpi
index 4f41e83..c9c4db2 100644
Binary files a/canvasblocker.xpi and b/canvasblocker.xpi differ
diff --git a/data/inject.js b/data/inject.js
index 0d4eeda..5601b26 100644
--- a/data/inject.js
+++ b/data/inject.js
@@ -147,7 +147,7 @@
 		toDataURL: {
 			mode: blockMode.readAPI,
 			object: unsafeWindow.HTMLCanvasElement,
-			fake: function(){
+			fake: function toDataURL(){
 				var type = arguments[0] || "image/png";
 				return "data:" + type + ";base64," + btoa(randomImage);
 			}
@@ -155,7 +155,7 @@
 		toBlob: {
 			mode: blockMode.readAPI,
 			object: unsafeWindow.HTMLCanvasElement,
-			fake: function(callback){
+			fake: function toBlob(callback){
 				var type = arguments[0] || "image/png";
 				var blob = new window.Blob(randomImage, {type: type});
 				callback(blob);
@@ -169,7 +169,7 @@
 		getImageData: {
 			mode: blockMode.readAPI,
 			object: unsafeWindow.CanvasRenderingContext2D,
-			fake: function(sx, sy, sw, sh){
+			fake: function getImageData(sx, sy, sw, sh){
 				var l = sw * sh * 4;
 				var data = new Uint8ClampedArray(l);
 				for (var i = 0; i < l; i += 1){
@@ -185,7 +185,7 @@
 		readPixels: {
 			mode: blockMode.readAPI,
 			object: unsafeWindow.WebGLRenderingContext,
-			fake: function(x, y, width, height, format, type, pixels){
+			fake: function readPixels(x, y, width, height, format, type, pixels){
 				// fake not working due to XRay copy restrictions...
 				// for (var i = 0; i < pixels.length; i += 1){
 					// pixels[i] = Math.floor(
diff --git a/lib/disableWithoutDocumentElement.js b/lib/disableWithoutDocumentElement.js
new file mode 100644
index 0000000..9fdedfb
--- /dev/null
+++ b/lib/disableWithoutDocumentElement.js
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+(function(){
+	 "use strict";
+	
+	const observers = require("sdk/system/events");
+	const { when: unload } = require("sdk/system/unload");
+	
+	var classes = {
+		HTMLCanvasElement: ["getContext", "toDataURL", "toBlob", "mozGetAsFile"],
+		CanvasRenderingContext2D: ["getImageData"],
+		WebGLRenderingContext: ["readPixels"]
+	};
+	
+	var classNames = Object.keys(classes);
+	
+	var originalProperties = new WeakMap();
+	
+	function disable({subject: window}){
+		var oldProperties = {};
+		classNames.forEach(function(className){
+			oldProperties[className] = {};
+			classes[className].forEach(function(funcName){
+				oldProperties[className][funcName] = window.wrappedJSObject[className].prototype[funcName];
+				window.wrappedJSObject[className].prototype[funcName] = function(){};
+			});
+		});
+		originalProperties.set(window, oldProperties);
+	}
+	
+	function reset({subject: document}){
+		var window = document.defaultView;
+		var oldProperties = originalProperties.get(window);
+		if (oldProperties){
+			originalProperties.delete(window);
+			classNames.forEach(function(className){
+				classes[className].forEach(function(funcName){
+					window.wrappedJSObject[className].prototype[funcName] = oldProperties[className][funcName];
+				});
+			});
+		}
+	}
+	
+	observers.on("content-document-global-created", disable);
+	unload(() => observers.off("content-document-global-created", disable));
+	observers.on("document-element-inserted", reset);
+	unload(() => observers.off("document-element-inserted", reset));
+}());
\ No newline at end of file
diff --git a/lib/main.js b/lib/main.js
index 4102195..47d8931 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -5,6 +5,7 @@
 (function(){
 	"use strict";
 	require("./stylePreferencePane");
+	require("./disableWithoutDocumentElement");
 	
 	var self = require("sdk/self");
 	var pageMod = require("sdk/page-mod");
@@ -70,7 +71,7 @@
 			var url = new URL(worker.url);
 			if (
 				(url.protocol === "about:") ||
-				(prefs.allowPDFCanvas && worker.tab.contentType.match(/\/pdf$/i))
+				(prefs.allowPDFCanvas && worker.tab && worker.tab.contentType.match(/\/pdf$/i))
 			){
 				mode = "unblock";
 			}