diff --git a/lib/traverse.js b/lib/traverse.js
index c16938a..020486a 100644
--- a/lib/traverse.js
+++ b/lib/traverse.js
@@ -84,7 +84,7 @@ function Traverse (refObj) {
             
             // use return values to update if defined
             var ret = f.call(state, node);
-            if (ret !== undefined) state.update(ret);
+            if (ret !== undefined && state.update) state.update(ret);
             
             if (typeof state.node == 'object'
             && state.node !== null && !state.circular) {
diff --git a/test/interface.js b/test/interface.js
new file mode 100755
index 0000000..2fe249a
--- /dev/null
+++ b/test/interface.js
@@ -0,0 +1,42 @@
+#!/usr/bin/env node
+var sys = require('sys');
+var Traverse = require('traverse');
+
+exports['interface map'] = function (assert) {
+    var obj = { a : [ 5,6,7 ], b : { c : [8] } };
+    
+    assert.equal(
+        Traverse.paths(obj)
+            .sort()
+            .map(function (path) { return path.join('/') })
+            .slice(1)
+            .join(' ')
+         ,
+         'a a/0 a/1 a/2 b b/c b/c/0'
+    );
+    
+    assert.equal(
+        Traverse.nodes(obj)
+            .map(function (node) { return sys.inspect(node) })
+            .join(' ')
+        ,
+        '{ a: [ 5, 6, 7 ], b: { c: [ 8 ] } } [ 5, 6, 7 ] 5 6 7 { c: [ 8 ] } [ 8 ] 8'
+    );
+    
+    assert.equal(
+        sys.inspect(Traverse.map(obj, function (node) {
+            if (typeof node == 'number') {
+                return node + 1000;
+            }
+            else if (Array.isArray(node)) {
+                return node.join(' ');
+            }
+        })),
+        "{ a: '5 6 7', b: { c: '8' } }"
+    );
+    
+    var nodes = 0;
+    Traverse.forEach(obj, function (node) { nodes ++ });
+    assert.equal(nodes, 8);
+};
+