diff --git a/packages/babel-types/src/definitions/core.js b/packages/babel-types/src/definitions/core.js
index 7a869a82f4..aab8ca2d2d 100644
--- a/packages/babel-types/src/definitions/core.js
+++ b/packages/babel-types/src/definitions/core.js
@@ -626,7 +626,7 @@ defineType("UnaryExpression", {
   builder: ["operator", "argument", "prefix"],
   fields: {
     prefix: {
-      default: false
+      default: true
     },
     argument: {
       validate: assertNodeType("Expression")