diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index fac701132a..3e13d640e0 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -2,14 +2,25 @@ var _ = require("lodash");
 
 var t = exports;
 
+var addAssert = function (type, is) {
+  t["assert" + type] = function (node, opts) {
+    opts = opts || {};
+    if (!is(node, opts)) {
+      throw new Error("Expected type " + JSON.stringify(type) + " with option " + JSON.stringify(opts));
+    }
+  };
+};
+
 //
 
 t.VISITOR_KEYS = require("./visitor-keys");
 
 _.each(t.VISITOR_KEYS, function (keys, type) {
-  t["is" + type] = function (node, opts) {
+  var is = t["is" + type] = function (node, opts) {
     return node && node.type === type && t.shallowEqual(node, opts);
   };
+
+  addAssert(type, is);
 });
 
 //
@@ -43,17 +54,21 @@ _.each(t.ALIAS_KEYS, function (aliases, type) {
 _.each(_aliases, function (types, type) {
   t[type.toUpperCase() + "_TYPES"] = types;
 
-  t["is" + type] = function (node, opts) {
+  var is = t["is" + type] = function (node, opts) {
     return node && _.contains(types, node.type) && t.shallowEqual(node, opts);
   };
+
+  addAssert(type, is);
 });
 
 //
 
 t.isExpression = function (node) {
-  return !t.isDeclaration(node);
+  return !t.isStatement(node);
 };
 
+addAssert("Expression", t.isExpression);
+
 //
 
 t.shallowEqual = function (actual, expected) {