diff --git a/HISTORY.md b/HISTORY.md
index edd6c05..e463052 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,3 +1,8 @@
+unreleased
+==========
+
+  * Fix handling of modified headers with invalid dates
+
 0.5.0 / 2017-02-21
 ==================
 
diff --git a/index.js b/index.js
index bd9812e..dd49dee 100644
--- a/index.js
+++ b/index.js
@@ -70,7 +70,7 @@ function fresh (reqHeaders, resHeaders) {
   // if-modified-since
   if (modifiedSince) {
     var lastModified = resHeaders['last-modified']
-    var modifiedStale = !lastModified || Date.parse(lastModified) > Date.parse(modifiedSince)
+    var modifiedStale = !lastModified || !(parseHttpDate(lastModified) <= parseHttpDate(modifiedSince))
 
     if (modifiedStale) {
       return false
@@ -79,3 +79,19 @@ function fresh (reqHeaders, resHeaders) {
 
   return true
 }
+
+/**
+ * Parse an HTTP Date into a number.
+ *
+ * @param {string} date
+ * @private
+ */
+
+function parseHttpDate (date) {
+  var timestamp = date && Date.parse(date)
+
+  // istanbul ignore next: guard against date.js Date.parse patching
+  return typeof timestamp === 'number'
+    ? timestamp
+    : NaN
+}
diff --git a/test/fresh.js b/test/fresh.js
index cd7f72d..f5e66ac 100644
--- a/test/fresh.js
+++ b/test/fresh.js
@@ -107,15 +107,15 @@ describe('fresh(reqHeaders, resHeaders)', function () {
     describe('with invalid If-Modified-Since date', function () {
       it('should be stale', function () {
         var reqHeaders = { 'if-modified-since': 'foo' }
-        var resHeaders = { 'modified-since': 'Sat, 01 Jan 2000 00:00:00 GMT' }
+        var resHeaders = { 'last-modified': 'Sat, 01 Jan 2000 00:00:00 GMT' }
         assert.ok(!fresh(reqHeaders, resHeaders))
       })
     })
 
-    describe('with invalid Modified-Since date', function () {
+    describe('with invalid Last-Modified date', function () {
       it('should be stale', function () {
         var reqHeaders = { 'if-modified-since': 'Sat, 01 Jan 2000 00:00:00 GMT' }
-        var resHeaders = { 'modified-since': 'foo' }
+        var resHeaders = { 'last-modified': 'foo' }
         assert.ok(!fresh(reqHeaders, resHeaders))
       })
     })