diff --git a/cssselect/xpath.py b/cssselect/xpath.py
index a8722bb..0c4daa5 100644
--- a/cssselect/xpath.py
+++ b/cssselect/xpath.py
@@ -326,6 +326,12 @@ def xpath_class(self, class_selector):
     def xpath_hash(self, id_selector):
         """Translate an ID selector."""
         xpath = self.xpath(id_selector.selector)
+        is_valid_identifier = not (re.match('--', id_selector.id)
+                                   or re.match('[0-9]', id_selector.id)
+                                   or re.match('-[0-9]', id_selector.id)
+                                   )
+        if not is_valid_identifier:
+            raise ExpressionError("invalid identifier")
         return self.xpath_attrib_equals(xpath, '@id', id_selector.id)
 
     def xpath_element(self, selector):
diff --git a/tests/test_cssselect.py b/tests/test_cssselect.py
index d6969f2..349a8d4 100644
--- a/tests/test_cssselect.py
+++ b/tests/test_cssselect.py
@@ -526,6 +526,9 @@ def xpath(css):
         self.assertRaises(ExpressionError, xpath, ':lorem-ipsum')
         self.assertRaises(ExpressionError, xpath, ':lorem(ipsum)')
         self.assertRaises(ExpressionError, xpath, '::lorem-ipsum')
+        self.assertRaises(ExpressionError, xpath, '#00FF55')
+        self.assertRaises(ExpressionError, xpath, '#--abc')
+        self.assertRaises(ExpressionError, xpath, '#-1abc')
         self.assertRaises(TypeError, GenericTranslator().css_to_xpath, 4)
         self.assertRaises(TypeError, GenericTranslator().selector_to_xpath,
             'foo')