diff --git a/packages/babel-parser/src/plugins/typescript/index.js b/packages/babel-parser/src/plugins/typescript/index.js
index 7ebe0ad44420..8bf6e47716a2 100644
--- a/packages/babel-parser/src/plugins/typescript/index.js
+++ b/packages/babel-parser/src/plugins/typescript/index.js
@@ -921,7 +921,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           // : asserts foo
           const node = this.startNodeAtNode(t);
           node.parameterName = this.parseIdentifier();
-          node.assertsModifier = assertsModifier;
+          node.assertsModifier = true;
           t.typeAnnotation = this.finishNode(node, "TSTypePredicate");
           return this.finishNode(t, "TSTypeAnnotation");
         }