diff --git a/index.js b/index.js
index d3fea32..f3db681 100644
--- a/index.js
+++ b/index.js
@@ -259,7 +259,11 @@ module.exports = function serialize(obj, options) {
         if (type === 'R') {
             // Sanitize flags to prevent code injection (only allow valid RegExp flag characters)
             var flags = String(regexps[valueIndex].flags).replace(/[^gimsuydv]/g, '');
-            return "new RegExp(" + serialize(regexps[valueIndex].source) + ", \"" + flags + "\")";
+            var regexpSource = regexps[valueIndex].source;
+            if (typeof regexpSource !== 'string') {
+                throw new TypeError('RegExp.source must be a string');
+            }
+            return "new RegExp(" + serialize(regexpSource) + ", \"" + flags + "\")";
         }
 
         if (type === 'M') {
diff --git a/test/unit/serialize.js b/test/unit/serialize.js
index b4eea83..2fc5709 100644
--- a/test/unit/serialize.js
+++ b/test/unit/serialize.js
@@ -316,6 +316,41 @@ describe('serialize( obj )', function () {
             strictEqual(serialize(re), 'new RegExp("[\\u003C\\\\\\u002Fscript\\u003E\\u003Cscript\\u003Ealert(\'xss\')\\\\\\u002F\\\\\\u002F]", "")');
         });
 
+        it('should throw when serializing a spoofed RegExp with non-string source', function () {
+            var fakeRe = Object.create(RegExp.prototype);
+            Object.defineProperty(fakeRe, 'source', {
+                enumerable: true,
+                value: {
+                    toString: function () {
+                        global.__RE_SOURCE_EXECUTED__ = 'pwned-regexp';
+                        return 'abc';
+                    }
+                }
+            });
+            Object.defineProperty(fakeRe, 'flags', { enumerable: true, value: '' });
+            throws(function () { serialize({ re: fakeRe }); }, TypeError);
+            strictEqual(global.__RE_SOURCE_EXECUTED__, undefined);
+        });
+
+        it('should throw when serializing a spoofed RegExp with non-string source via getter', function () {
+            var fakeRegex = Object.create(RegExp.prototype);
+            Object.defineProperty(fakeRegex, 'source', {
+                get: function () {
+                    return {
+                        toString: function () {
+                            global.__REGEXP_SOURCE_GETTER_EXECUTED__ = 'pwned';
+                            return 'x';
+                        }
+                    };
+                }
+            });
+            Object.defineProperty(fakeRegex, 'flags', {
+                get: function () { return 'g'; }
+            });
+            throws(function () { serialize({ re: fakeRegex }); }, TypeError);
+            strictEqual(global.__REGEXP_SOURCE_GETTER_EXECUTED__, undefined);
+        });
+
         it('should sanitize RegExp.flags to prevent code injection', function () {
             // Object that passes instanceof RegExp with attacker-controlled .flags
             var fakeRegex = Object.create(RegExp.prototype);