diff --git a/packages/babel-generator/src/generators/flow.js b/packages/babel-generator/src/generators/flow.js
index decdde9d74..b20ebe593a 100644
--- a/packages/babel-generator/src/generators/flow.js
+++ b/packages/babel-generator/src/generators/flow.js
@@ -104,7 +104,7 @@ export function _interfaceish(node: Object) {
     this.push(" extends ");
     this.printJoin(node.extends, node, { separator: ", " });
   }
-  if (node.mixins.length) {
+  if (node.mixins && node.mixins.length) {
     this.push(" mixins ");
     this.printJoin(node.mixins, node, { separator: ", " });
   }