diff --git a/index.js b/index.js
index bfa9819..4186320 100644
--- a/index.js
+++ b/index.js
@@ -13,4 +13,52 @@ function readYAML(file) {
     return yaml.eval(data);
 }
 
+// uap-ref-impl does not provide a `toVersionString` function. Add it.
+function ensureToVersionString(context) {
+  if (context && ! context.toVersionString) {
+    context.toVersionString = toVersionString.bind(context);
+  }
+}
+
+// taken from https://github.com/tobie/ua-parser/blob/8bc22f74e3a4515633d975cd535e94e5ae9699d2/js/lib/os.js#L12
+function toVersionString () {
+  var output = '';
+  if (this.major != null) {
+    output += this.major;
+    if (this.minor != null) {
+      output += '.' + this.minor;
+      if (this.patch != null) {
+        if (startsWithDigit(this.patch)) {
+          output += '.';
+        }
+        output += this.patch;
+        if (this.patchMinor != null) {
+          if (startsWithDigit(this.patchMinor)) {
+            output += '.';
+          }
+          output += this.patchMinor;
+        }
+      }
+    }
+  }
+
+  return output;
+}
+
+function startsWithDigit(str) {
+  return /^\d/.test(str);
+}
+
+var _parse = refImpl.parse;
+refImpl.parse = function (ua) {
+  var result = _parse.call(refImpl, ua);
+
+  // ua-parser results had a `toVersionString` function on `os` and `ua`.
+  // Add the missing support.
+  ensureToVersionString(result.os);
+  ensureToVersionString(result.ua);
+
+  return result;
+};
+
 module.exports = refImpl;