diff --git a/lib/mozilla-ast.js b/lib/mozilla-ast.js
index 3e0534c8..18e628b7 100644
--- a/lib/mozilla-ast.js
+++ b/lib/mozilla-ast.js
@@ -668,6 +668,7 @@
             type: "ArrowFunctionExpression",
             async: is_async(M),
             params: params,
+            expression: !!M.value,
             body: M.value ? to_moz(M.value) : to_moz_scope("BlockStatement", M),
         };
         return {