Improved validation as per suggestions from @cjihrig

README.md

@@ -31,8 +31,8 @@ Lead Maintainer - [Eran Hammer](https://github.com/hueniverse)
             - [`null()`](#null)
             - [`undefined()`](#undefined)
         - [`include(values)`](#includevalues)
-        - [`startWith(value)`](#startWith)
-        - [`endWith(value)`](#endWith)
+        - [`startWith(value)`](#startwithvalue)
+        - [`endWith(value)`](#startwithvalue)
         - [`exist()`](#exist)
         - [`empty()`](#empty)
         - [`length(size)`](#lengthsize)

lib/index.js

@@ -121,12 +121,16 @@ internals.addMethod(['include', 'includes', 'contain', 'contains'], function (va
 
 internals.addMethod(['endWith', 'endsWith'], function (value) {
 
+    internals.assert(this, typeof this._ref === 'string' && typeof value === 'string', 'Can only assert endsWith on a string, with a string');
+
     var comparator = this._ref.slice(-Math.abs(value.length));
     return this.assert(comparator === value, 'endWith ' + internals.display(value));
 });
 
 internals.addMethod(['startWith', 'startsWith'], function (value) {
 
+    internals.assert(this, typeof this._ref === 'string' && typeof value === 'string', 'Can only assert startsWith on a string, with a string');
+
     var comparator = this._ref.slice(0, value.length);
     return this.assert(comparator === value, 'startWith ' + internals.display(value));
 });

test/index.js

@@ -729,6 +729,34 @@ describe('expect()', function () {
                 done();
             });
 
+            it('does not validate arrays', function (done) {
+
+                var exception = false;
+                try {
+                    Code.expect(['a', 'b', 'c']).to.endWith('abcdef');
+                }
+                catch (err) {
+                    exception = err;
+                }
+
+                Hoek.assert(exception.message === 'Can only assert endsWith on a string, with a string', exception);
+                done();
+            });
+
+            it('does not validate using arrays', function (done) {
+
+                var exception = false;
+                try {
+                    Code.expect('abcdef').to.endWith(['a', 'b', 'c']);
+                }
+                catch (err) {
+                    exception = err;
+                }
+
+                Hoek.assert(exception.message === 'Can only assert endsWith on a string, with a string', exception);
+                done();
+            });
+
         });
 
         describe('startWith()', function () {
@@ -752,6 +780,34 @@ describe('expect()', function () {
                 done();
             });
 
+            it('does not validate arrays', function (done) {
+
+                var exception = false;
+                try {
+                    Code.expect(['a', 'b', 'c']).to.startWith('abcdef');
+                }
+                catch (err) {
+                    exception = err;
+                }
+
+                Hoek.assert(exception.message === 'Can only assert startsWith on a string, with a string', exception);
+                done();
+            });
+
+            it('does not validate using arrays', function (done) {
+
+                var exception = false;
+                try {
+                    Code.expect('abcdef').to.startWith(['a', 'b', 'c']);
+                }
+                catch (err) {
+                    exception = err;
+                }
+
+                Hoek.assert(exception.message === 'Can only assert startsWith on a string, with a string', exception);
+                done();
+            });
+
         });
 
         describe('exist()', function () {