diff --git a/lib/6to5/transformation/transform.js b/lib/6to5/transformation/transform.js
index e0238cc5f0..a98b0fe820 100644
--- a/lib/6to5/transformation/transform.js
+++ b/lib/6to5/transformation/transform.js
@@ -98,6 +98,7 @@ _.each({
   typeofSymbol:              require("./transformers/optional-typeof-symbol"),
   coreAliasing:              require("./transformers/optional-core-aliasing"),
   undefinedToVoid:           require("./transformers/optional-undefined-to-void"),
+  undeclaredVariableCheck:   require("./transformers/optional-undeclared-variable-check"),
 
   // spec
   specPropertyLiterals:         require("./transformers/spec-property-literals"),
diff --git a/lib/6to5/transformation/transformers/optional-undeclared-variable-check.js b/lib/6to5/transformation/transformers/optional-undeclared-variable-check.js
new file mode 100644
index 0000000000..6640c0217f
--- /dev/null
+++ b/lib/6to5/transformation/transformers/optional-undeclared-variable-check.js
@@ -0,0 +1,9 @@
+var t = require("../../types");
+
+exports.optional = true;
+
+exports.Identifier = function (node, parent, scope, context, file) {
+  if (!scope.has(node.name, true)) {
+    throw file.errorWithNode(node, "Reference to undeclared variable");
+  }
+};
diff --git a/test/fixtures/transformation/optional-undeclared-variable-check/declared/exec.js b/test/fixtures/transformation/optional-undeclared-variable-check/declared/exec.js
new file mode 100644
index 0000000000..e2721feb4a
--- /dev/null
+++ b/test/fixtures/transformation/optional-undeclared-variable-check/declared/exec.js
@@ -0,0 +1,5 @@
+function foo() {
+  
+}
+
+foo();
diff --git a/test/fixtures/transformation/optional-undeclared-variable-check/options.json b/test/fixtures/transformation/optional-undeclared-variable-check/options.json
new file mode 100644
index 0000000000..535d39109b
--- /dev/null
+++ b/test/fixtures/transformation/optional-undeclared-variable-check/options.json
@@ -0,0 +1,3 @@
+{
+  "optional": ["undeclaredVariableCheck"]
+}
diff --git a/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/exec.js b/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/exec.js
new file mode 100644
index 0000000000..a280f9a5cc
--- /dev/null
+++ b/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/exec.js
@@ -0,0 +1 @@
+foo();
diff --git a/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/options.json b/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/options.json
new file mode 100644
index 0000000000..a661ffc703
--- /dev/null
+++ b/test/fixtures/transformation/optional-undeclared-variable-check/undeclared/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Reference to undeclared variable"
+}