Add docs and throw exception when type is invalid, test coverage

@@ -160,9 +160,9 @@ Asserts that a given subject is `falsy <http://11heavens.com/falsy-and-truthy-in
 ``assertField()``
 -------------------------------------------------------------------------------
 
-**Signature:** ``assertField(String|object input, String expected[, String message, Object options])``
+**Signature:** ``assertField(String|Object input, String expected[, String message, Object options])``
 
-Asserts that a given form field has the provided value::
+Asserts that a given form field has the provided value with input name or :ref:`selector expression <selectors>`::
 
     casper.test.begin('assertField() tests', 1, function(test) {
         casper.start('http://www.google.fr/', function() {

@@ -450,6 +450,7 @@ Tester.prototype.assertEvalEqual = function assertEvalEquals(fn, expected, messa
 };
 
 function baseFieldAssert(inputName, expected, actual, message) {
+    /*jshint validthis:true */
     "use strict";
 
     return this.assert(utils.equals(actual, expected),  message, {
@@ -503,6 +504,8 @@ Tester.prototype.assertField = function assertField(input, expected, message, op
                 return this.assertFieldCSS(input.path, expected, message);
             case 'xpath':
                 return this.assertFieldXPath(input.path, expected, message);
+            default:
+                throw new CasperError('Invalid regexp.');
             // no default
         }
     }

@@ -170,6 +170,25 @@ casper.test.begin('Tester.assertField(): XPath selectors', 1, function(test) {
     });
 });
 
+casper.test.begin('Tester.assertField(): invalid selectors', 1, function(test) {
+    casper.start('tests/site/form.html', function() {
+        this.fill('form[action="result.html"]', {
+            'email': 'albert@camus.com'
+        });
+
+        test.assertRaise(function() {
+            test.assertField({
+                type: 'albert'
+            },
+                 'albert@camus.com',
+                 'Tester.assertField() works as expected with XPath selectors'
+            );
+        }, [], 'should throw an error for an invalid selector');
+    }).run(function() {
+        test.done();
+    });
+});
+
 casper.test.begin('Tester.assertFieldCSS(): CSS selectors', 1, function(test) {
     casper.start('tests/site/form.html', function() {
         this.fill('form[action="result.html"]', {