Reject non-string expressions

test.js

@@ -1305,11 +1305,18 @@ describe('xpath', () => {
         it('should reject unspecified expression', () => {
             for (let expr of [null, undefined, '']) {
                 assert.throws(() => xpath.parse(expr), {
-                    message: 'XPath expression unspecified',
+                    message: 'XPath expression unspecified.',
                 });
             }
         });
 
+        it('should reject non-string expression', () => {
+            for (let expr of [{}, []]) {
+                assert.throws(() => xpath.parse(expr), {
+                    message: 'XPath expression must be a string.',
+                });
+            }
+        });
         it('should reject non-nodes', () => {
             for (let node of ['<n />', 0, 45, true, false, [], {}]) {
                 assert.throws(() => xpath.parse('/*').select({ node }), {

xpath.js

@@ -1269,7 +1269,10 @@ var xpath = (typeof exports === 'undefined') ? {} : exports;
 
     XPathParser.prototype.parse = function (s) {
         if (!s) {
-            throw new Error('XPath expression unspecified');
+            throw new Error('XPath expression unspecified.');
+        }
+        if (typeof s !== 'string'){
+            throw new Error('XPath expression must be a string.');
         }
 
         var types;