diff --git a/browser.js b/browser.js
index 344ecab..e0a1b17 100644
--- a/browser.js
+++ b/browser.js
@@ -103,7 +103,11 @@ function log() {
 
 function save(namespaces) {
   try {
-    localStorage.debug = namespaces;
+    if (null == namespaces) {
+      delete localStorage.debug;
+    } else {
+      localStorage.debug = namespaces;
+    }
   } catch(e) {}
 }
 
diff --git a/node.js b/node.js
index f54bcc0..772bb42 100644
--- a/node.js
+++ b/node.js
@@ -84,7 +84,13 @@ function log() {
  */
 
 function save(namespaces) {
-  process.env.DEBUG = namespaces;
+  if (null == namespaces) {
+    // If you set a process.env field to null or undefined, it gets cast to the
+    // string 'null' or 'undefined'. Just delete instead.
+    delete process.env.DEBUG;
+  } else {
+    process.env.DEBUG = namespaces;
+  }
 }
 
 /**