diff --git a/src/babel/transformation/transformers/other/flow.js b/src/babel/transformation/transformers/other/flow.js
index 821336f65b..c5cb1e0631 100644
--- a/src/babel/transformation/transformers/other/flow.js
+++ b/src/babel/transformation/transformers/other/flow.js
@@ -13,6 +13,13 @@ export function Class(node) {
   node.implements = null;
 }
 
+export function Function(node) {
+  for (var i = 0; i < node.params.length; i++) {
+    var param = node.params[i];
+    param.optional = false;
+  }
+}
+
 export function TypeCastExpression(node) {
   return node.expression;
 }
diff --git a/test/fixtures/transformation/flow/strip-type-annotations/expected.js b/test/fixtures/transformation/flow/strip-type-annotations/expected.js
index 91f40a7778..62bf507aae 100644
--- a/test/fixtures/transformation/flow/strip-type-annotations/expected.js
+++ b/test/fixtures/transformation/flow/strip-type-annotations/expected.js
@@ -61,7 +61,7 @@ class Foo7 {
 class Foo8 {
   "bar"() {}
 }
-function foo(requiredParam, optParam?) {}
+function foo(requiredParam, optParam) {}
 class Foo9 {}
 class Foo10 {}
 var x = 4;