diff --git a/lib/utils.js b/lib/utils.js
index 4f0be041..a0434d58 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -100,13 +100,24 @@ exports.escapeXML = function (markup) {
       .replace(_MATCH_HTML, encode_char);
 };
 
-// If the Object prototype is frozen, the "toString" property is non-writable. This means that any objects which inherit this property
-// cannot have the property changed using an assignment. If using strict mode, attempting that will cause an error. If not using strict
-// mode, attempting that will be silently ignored.
-// However, we can still explicitly shadow the prototype's "toString" property by defining a new "toString" property on this object.
-Object.defineProperty(exports.escapeXML, 'toString', function () {
+function escapeXMLToString() {
   return Function.prototype.toString.call(this) + ';\n' + escapeFuncStr;
-});
+}
+
+try {
+  if (typeof Object.defineProperty === 'function') {
+  // If the Function prototype is frozen, the "toString" property is non-writable. This means that any objects which inherit this property
+  // cannot have the property changed using an assignment. If using strict mode, attempting that will cause an error. If not using strict
+  // mode, attempting that will be silently ignored.
+  // However, we can still explicitly shadow the prototype's "toString" property by defining a new "toString" property on this object.
+    Object.defineProperty(exports.escapeXML, 'toString', { value: escapeXMLToString });
+  } else {
+    // If Object.defineProperty() doesn't exist, attempt to shadow this property using the assignment operator.
+    exports.escapeXML.toString = escapeXMLToString;
+  }
+} catch (err) {
+  console.warn('Unable to set escapeXML.toString (is the Function prototype frozen?)');
+}
 
 /**
  * Naive copy of properties from one object to another.
diff --git a/package.json b/package.json
index 047f6ee1..88b28a98 100644
--- a/package.json
+++ b/package.json
@@ -37,6 +37,6 @@
     "node": ">=0.10.0"
   },
   "scripts": {
-    "test": "mocha"
+    "test": "mocha -u tdd"
   }
 }
diff --git a/test/utils.js b/test/utils.js
index 327e9171..0dd56561 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -83,8 +83,27 @@ suite('unit testing exported functions of module \'utils.js\'', function () {
    */
   suite('unit testing function \'escapeXML\' of module \'utils.js\'', function () {
     test('it should be callable without parameters', function () {
+      const stringified =
+`function (markup) {
+  return markup == undefined
+    ? ''
+    : String(markup)
+      .replace(_MATCH_HTML, encode_char);
+};
+var _ENCODE_HTML_RULES = {
+      "&": "&"
+    , "<": "&lt;"
+    , ">": "&gt;"
+    , '"': "&#34;"
+    , "'": "&#39;"
+    }
+  , _MATCH_HTML = /[&<>'"]/g;
+function encode_char(c) {
+  return _ENCODE_HTML_RULES[c] || c;
+};
+`;
       assert.doesNotThrow(() => { utils.escapeXML.toString(); });
-      assert.ok(typeof(utils.escapeXML.toString())==='string');
+      assert.ok(utils.escapeXML.toString()===stringified);
     });
   });