Only check for direct properties from the headers object

src/main/ua-parser.js

@@ -1400,7 +1400,9 @@
                 // Normalize headers field name into lowercase
                 var normalized = {};
                 for (var header in headers) {
-                    normalized[String(header).toLowerCase()] = headers[header];
+                    if (headers.hasOwnProperty(header)) {
+                        normalized[String(header).toLowerCase()] = headers[header];
+                    }
                 }
                 headers = normalized;
             }

test/unit/main.js

@@ -389,4 +389,10 @@ describe('Read user-agent data from req.headers', function () {
         const { browser } = UAParser(hEaDeRs);
         assert.strictEqual(browser.toString(), "Midori 0.2.2");
     });
+
+    it('Empty headers should not raise any error', function () {    
+        const emptyHeaders = {};
+        const { browser } = UAParser(emptyHeaders);
+        assert.strictEqual(browser.toString(), "undefined");
+    });
 });