diff --git a/bits/20_jsutils.js b/bits/20_jsutils.js
index d12c438..2289e1d 100644
--- a/bits/20_jsutils.js
+++ b/bits/20_jsutils.js
@@ -76,7 +76,7 @@ function str2cc(str) {
 
 function dup(o/*:any*/)/*:any*/ {
 	if(typeof JSON != 'undefined') return JSON.parse(JSON.stringify(o));
-	if(typeof o != 'object' || !o) return o;
+	if(typeof o != 'object' || o == null) return o;
 	var out = {};
 	for(var k in o) if(o.hasOwnProperty(k)) out[k] = dup(o[k]);
 	return out;
diff --git a/xlsx.flow.js b/xlsx.flow.js
index 1a05868..95f8e80 100644
--- a/xlsx.flow.js
+++ b/xlsx.flow.js
@@ -1399,7 +1399,7 @@ function str2cc(str) {
 
 function dup(o/*:any*/)/*:any*/ {
 	if(typeof JSON != 'undefined') return JSON.parse(JSON.stringify(o));
-	if(typeof o != 'object' || !o) return o;
+	if(typeof o != 'object' || o == null) return o;
 	var out = {};
 	for(var k in o) if(o.hasOwnProperty(k)) out[k] = dup(o[k]);
 	return out;
diff --git a/xlsx.js b/xlsx.js
index 7c0ac53..dcc7a8d 100644
--- a/xlsx.js
+++ b/xlsx.js
@@ -1353,7 +1353,7 @@ function str2cc(str) {
 
 function dup(o) {
 	if(typeof JSON != 'undefined') return JSON.parse(JSON.stringify(o));
-	if(typeof o != 'object' || !o) return o;
+	if(typeof o != 'object' || o == null) return o;
 	var out = {};
 	for(var k in o) if(o.hasOwnProperty(k)) out[k] = dup(o[k]);
 	return out;