diff --git a/lib/model.js b/lib/model.js
index e25c42b..905a9e3 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -287,7 +287,10 @@ function areThingsEqual (a, b) {
 }
 
 
-comparisonFunctions.$lt = function (a, b) {
+/**
+ * Check that two values are comparable
+ */
+function areComparable (a, b) {
   if (typeof a !== 'string' && typeof a !== 'number' && !util.isDate(a) &&
       typeof b !== 'string' && typeof b !== 'number' && !util.isDate(b)) {
     return false;
@@ -295,7 +298,12 @@ comparisonFunctions.$lt = function (a, b) {
 
   if (typeof a !== typeof b) { return false; }
 
-  return a < b;
+  return true;
+}
+
+
+comparisonFunctions.$lt = function (a, b) {
+  return areComparable(a, b) && a < b;
 };