diff --git a/boot/boot.js b/boot/boot.js
index aa1e0f5bd..f9f8d16f2 100644
--- a/boot/boot.js
+++ b/boot/boot.js
@@ -793,7 +793,7 @@ $tw.Tiddler = function(/* [fields,] fields */) {
 					value = src[t];
 				}
 				// Freeze the field to keep it immutable
-				if(typeof value === "object") {
+				if(value != null && typeof value === "object") {
 					Object.freeze(value);
 				}
 				this.fields[t] = value;