diff --git a/src/compiler/factory.ts b/src/compiler/factory.ts
index dfc22e3183374..2722a3394fd0f 100644
--- a/src/compiler/factory.ts
+++ b/src/compiler/factory.ts
@@ -28,6 +28,8 @@ namespace ts {
             if (isNodeArray(elements)) {
                 return elements;
             }
+            // elements is a ReadonlyArray, so create a fresh array before adding properties.
+            elements = elements.slice();
         }
         else {
             elements = [];