diff --git a/jmespath/visitor.py b/jmespath/visitor.py
index 15fb1774..d9fbba0b 100644
--- a/jmespath/visitor.py
+++ b/jmespath/visitor.py
@@ -35,12 +35,18 @@ def _is_special_number_case(x, y):
         return isinstance(x, bool)
 
 
-def _is_comparable(x):
+def _is_comparable(x, y):
+    # Ordering operators are only valid when both operands
+    # belong to the same comparable category, i.e. both are
+    # numbers or both are strings.  Mixing categories (e.g. a
+    # number and a string) is not comparable and yields null.
+    #
     # The spec doesn't officially support string types yet,
     # but enough people are relying on this behavior that
     # it's been added back.  This should eventually become
     # part of the official spec.
-    return _is_actual_number(x) or isinstance(x, string_type)
+    return ((_is_actual_number(x) and _is_actual_number(y)) or
+            (isinstance(x, string_type) and isinstance(y, string_type)))
 
 
 def _is_actual_number(x):
@@ -151,9 +157,7 @@ def visit_comparator(self, node, value):
             # will yield a None value.
             left = self.visit(node['children'][0], value)
             right = self.visit(node['children'][1], value)
-            num_types = (int, float)
-            if not (_is_comparable(left) and
-                    _is_comparable(right)):
+            if not _is_comparable(left, right):
                 return None
             return comparator_func(left, right)
 
diff --git a/tests/compliance/boolean.json b/tests/compliance/boolean.json
index dd7ee588..891494c4 100644
--- a/tests/compliance/boolean.json
+++ b/tests/compliance/boolean.json
@@ -220,7 +220,8 @@
       "two": 2,
       "three": 3,
       "emptylist": [],
-      "boolvalue": false
+      "boolvalue": false,
+      "stringvalue": "2"
     },
     "cases": [
       {
@@ -267,6 +268,26 @@
         "expression": "one < boolvalue",
         "result": null
       },
+      {
+        "expression": "one < stringvalue",
+        "result": null
+      },
+      {
+        "expression": "one <= stringvalue",
+        "result": null
+      },
+      {
+        "expression": "one > stringvalue",
+        "result": null
+      },
+      {
+        "expression": "one >= stringvalue",
+        "result": null
+      },
+      {
+        "expression": "stringvalue < one",
+        "result": null
+      },
       {
         "expression": "one < two && three > one",
         "result": true