diff --git a/__tests__/mainTest.js b/__tests__/mainTest.js
index 4097053..80c42de 100644
--- a/__tests__/mainTest.js
+++ b/__tests__/mainTest.js
@@ -41,7 +41,7 @@ test('begin event', () => {
         });
         NativeEventEmitter.listeners.downloadBegin({
             id: 'testBegin',
-            expctedBytes: 9001
+            expectedBytes: 9001
         });
     });
 });
@@ -159,17 +159,17 @@ test('wrong handler type', () => {
 
     expect(() => {
         dt.begin('not function');
-    }).toThrow('handler must be a function');
+    }).toThrow();
 
     expect(() => {
         dt.progress(7);
-    }).toThrow('handler must be a function');
+    }).toThrow();
 
     expect(() => {
         dt.done({iamnota: 'function'});
-    }).toThrow('handler must be a function');
+    }).toThrow();
 
     expect(() => {
         dt.error('not function');
-    }).toThrow('handler must be a function');
+    }).toThrow();
 });
\ No newline at end of file
diff --git a/index.js b/index.js
index c572c3e..492114e 100644
--- a/index.js
+++ b/index.js
@@ -53,6 +53,7 @@ export function checkForExistingDownloads() {
                     if (taskInfo.bytesWritten === taskInfo.totalBytes) {
                         task.state = 'DONE';
                     } else {
+                        // IOS completed the download but it was not done.
                         return null;
                     }
                 }
diff --git a/lib/downloadTask.js b/lib/downloadTask.js
index 6b15d33..72e5a39 100644
--- a/lib/downloadTask.js
+++ b/lib/downloadTask.js
@@ -1,6 +1,11 @@
 import { NativeModules } from 'react-native';
 const { RNBackgroundDownload } = NativeModules;
 
+function validateHandler(handler) {
+    if (!(typeof handler === 'function')) {
+        throw new TypeError(`[RNBackgroundDownload] expected argument to be a function, got: ${typeof handler}`);
+    }
+}
 export default class DownloadTask {
     state = 'PENDING'
     percent = 0
@@ -19,33 +24,25 @@ export default class DownloadTask {
     }
 
     begin(handler) {
-        if (!(typeof handler === 'function')) {
-            throw new TypeError('handler must be a function');
-        }
+        validateHandler(handler);
         this._beginHandler = handler;
         return this;
     }
 
     progress(handler) {
-        if (!(typeof handler === 'function')) {
-            throw new TypeError('handler must be a function');
-        }
+        validateHandler(handler);
         this._progressHandler = handler;
         return this;
     }
 
     done(handler) {
-        if (!(typeof handler === 'function')) {
-            throw new TypeError('handler must be a function');
-        }
+        validateHandler(handler);
         this._doneHandler = handler;
         return this;
     }
 
     error(handler) {
-        if (!(typeof handler === 'function')) {
-            throw new TypeError('handler must be a function');
-        }
+        validateHandler(handler);
         this._errorHandler = handler;
         return this;
     }