diff --git a/src/api.js b/src/api.js
index 2ab23bdde..b3ff6f8d6 100644
--- a/src/api.js
+++ b/src/api.js
@@ -18,7 +18,9 @@
  * @return {Promise} A promise that is resolved with {PDFDocumentProxy} object.
  */
 PDFJS.getDocument = function getDocument(source) {
-  var url, data, headers, password, parameters = {};
+  var url, data, headers, password, parameters = {}, workerInitializedPromise,
+    workerReadyPromise, transport;
+
   if (typeof source === 'string') {
     url = source;
   } else if (isArrayBuffer(source)) {
@@ -37,8 +39,9 @@ PDFJS.getDocument = function getDocument(source) {
           'string or a parameter object');
   }
 
-  var promise = new PDFJS.Promise();
-  var transport = new WorkerTransport(promise);
+  workerInitializedPromise = new PDFJS.Promise();
+  workerReadyPromise = new PDFJS.Promise();
+  transport = new WorkerTransport(workerInitializedPromise, workerReadyPromise);
   if (data) {
     // assuming the data is array, instantiating directly from it
     transport.sendData(data, parameters);
@@ -48,24 +51,31 @@ PDFJS.getDocument = function getDocument(source) {
       {
         url: url,
         progress: function getPDFProgress(evt) {
-          if (evt.lengthComputable)
-            promise.progress({
+          if (evt.lengthComputable) {
+            workerReadyPromise.progress({
               loaded: evt.loaded,
               total: evt.total
             });
+          }
         },
         error: function getPDFError(e) {
-          promise.reject('Unexpected server response of ' +
+          workerReadyPromise.reject('Unexpected server response of ' +
             e.target.status + '.');
         },
         headers: headers
       },
       function getPDFLoad(data) {
-        transport.sendData(data, parameters);
+        // sometimes the pdf has finished downloading before the web worker-test
+        // has finished. In that case the rendering of the final pdf would cause
+        // errors. We have to wait for the WorkerTransport to finalize worker-
+        // support detection
+        workerInitializedPromise.then(function workerInitialized() {
+          transport.sendData(data, parameters);
+        });
       });
   }
 
-  return promise;
+  return workerReadyPromise;
 };
 
 /**
@@ -426,8 +436,8 @@ var PDFPageProxy = (function PDFPageProxyClosure() {
  * For internal use only.
  */
 var WorkerTransport = (function WorkerTransportClosure() {
-  function WorkerTransport(promise) {
-    this.workerReadyPromise = promise;
+  function WorkerTransport(workerInitializedPromise, workerReadyPromise) {
+    this.workerReadyPromise = workerReadyPromise;
     this.objs = new PDFObjects();
 
     this.pageCache = [];
@@ -471,6 +481,7 @@ var WorkerTransport = (function WorkerTransportClosure() {
             globalScope.PDFJS.disableWorker = true;
             this.setupFakeWorker();
           }
+          workerInitializedPromise.resolve();
         }.bind(this));
 
         var testObj = new Uint8Array(1);
@@ -486,6 +497,7 @@ var WorkerTransport = (function WorkerTransportClosure() {
     // Thus, we fallback to a faked worker.
     globalScope.PDFJS.disableWorker = true;
     this.setupFakeWorker();
+    workerInitializedPromise.resolve();
   }
   WorkerTransport.prototype = {
     destroy: function WorkerTransport_destroy() {
diff --git a/web/compatibility.js b/web/compatibility.js
index 528841bb6..4b7119c63 100644
--- a/web/compatibility.js
+++ b/web/compatibility.js
@@ -6,6 +6,16 @@
 // Checking if the typed arrays are supported
 (function checkTypedArrayCompatibility() {
   if (typeof Uint8Array !== 'undefined') {
+    // some mobile versions do not support subarray (e.g. safari 5 / iOS)
+    if (typeof Uint8Array.prototype.subarray === 'undefined') {
+        Uint8Array.prototype.subarray = function subarray(start, end) {
+          return new Uint8Array(this.slice(start, end));
+        };
+        Float32Array.prototype.subarray = function subarray(start, end) {
+          return new Float32Array(this.slice(start, end));
+        };
+    }
+
     // some mobile version might not support Float64Array
     if (typeof Float64Array === 'undefined')
       window.Float64Array = Float32Array;
@@ -69,8 +79,17 @@
 
 // Object.defineProperty() ?
 (function checkObjectDefinePropertyCompatibility() {
-  if (typeof Object.defineProperty !== 'undefined')
-    return;
+  if (typeof Object.defineProperty !== 'undefined') {
+    // some browsers (e.g. safari) cannot use defineProperty() on DOM objects
+    // and thus the native version is not sufficient
+    var definePropertyPossible = true;
+    try {
+      Object.defineProperty(new Image(), 'id', { value: 'test' });
+    } catch (e) {
+      definePropertyPossible = false;
+    }
+    if (definePropertyPossible) return;
+  }
 
   Object.defineProperty = function objectDefineProperty(obj, name, def) {
     delete obj[name];