diff --git a/bignumber.js b/bignumber.js
index a0debf3..ca980d9 100644
--- a/bignumber.js
+++ b/bignumber.js
@@ -196,7 +196,7 @@
 
         // Duplicate.
         if (n instanceof BigNumber 
-          || (n != null && n.s != null && n.e != null && n.c != null)) {
+          || (n && typeof n.s !== 'undefined' && typeof n.e !== 'undefined' && typeof n.c !== 'undefined')) {
           x.s = n.s;
           x.e = n.e;
           x.c = (n = n.c) ? n.slice() : n;
diff --git a/test/methods/BigNumber.js b/test/methods/BigNumber.js
index f1c70cc..3bbac1b 100644
--- a/test/methods/BigNumber.js
+++ b/test/methods/BigNumber.js
@@ -201,6 +201,7 @@ Test('bigNumber', function () {
 
     t('100002222.2222333322', new BigNumber({ s: 1, e: 8, c: [100002222, 22223333220000] }).toString())
     t('7777777777.123123123', new BigNumber({ s: 1, e: 9, c: [7777777777, 12312312300000] }).toString())
+    t('NaN', new BigNumber({ s: null, e: null, c: null }).toString())
 
     // Base-conversion tests