diff --git a/lib/6to5/transformers/let-scoping.js b/lib/6to5/transformers/let-scoping.js
index 7345cdd7a7..cd7a00efdc 100644
--- a/lib/6to5/transformers/let-scoping.js
+++ b/lib/6to5/transformers/let-scoping.js
@@ -26,8 +26,10 @@ exports.VariableDeclaration = function (node, parent, file) {
     if (util.isReferenced(node, parent)) return id;
   };
 
+  var isProgram = parent.type === "Program";
+
   var replace = function (node, parent) {
-    if (_.contains(traverse.FUNCTION_TYPES, node.type)) {
+    if (!isProgram && _.contains(traverse.FUNCTION_TYPES, node.type)) {
       var letReferences = [];
 
       traverse(node, function (node, parent) {
@@ -37,21 +39,12 @@ exports.VariableDeclaration = function (node, parent, file) {
       });
 
       if (letReferences.length) {
-        var callNode = function () {
+        if (node.type === "FunctionDeclaration") {
+          throw new Error("`FunctionDeclaration`s that use `let` and `constant` references aren't allowed outside of the root scope");
+        } else {
           return b.callExpression(b.functionExpression(null, letReferences, b.blockStatement([
             b.returnStatement(node)
           ])), letReferences);
-        };
-
-        if (node.type === "FunctionDeclaration") {
-          util.ensureExpressionType(node);
-          var declar = b.variableDeclaration("var", [
-            b.variableDeclarator(node.id, callNode())
-          ]);
-          declar._blockHoist = true;
-          return declar;
-        } else {
-          return callNode();
         }
       } else {
         return false;
diff --git a/test/fixtures/syntax/let-scoping/no-nested-function-declarations/actual.js b/test/fixtures/syntax/let-scoping/no-nested-function-declarations/actual.js
new file mode 100644
index 0000000000..dc5a13878d
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/no-nested-function-declarations/actual.js
@@ -0,0 +1,7 @@
+var numbers = [1, 2, 3];
+
+for (let i in numbers) {
+  function foo() {
+    return i;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/no-nested-function-declarations/options.json b/test/fixtures/syntax/let-scoping/no-nested-function-declarations/options.json
new file mode 100644
index 0000000000..ddd3d8df0f
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/no-nested-function-declarations/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "`FunctionDeclaration`s that use `let` and `constant` references aren't allowed outside of the root scope"
+}