diff --git a/lib/compress.js b/lib/compress.js
index d78d41d6..c1cb1464 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1430,7 +1430,7 @@ merge(Compressor.prototype, {
                     return node;
                 }
             });
-            tt.stack = compressor.stack; // so that's able to see parent nodes
+            tt.stack = compressor.stack.slice(); // so that's able to see parent nodes
             self = self.transform(tt);
         } catch(ex) {
             if (ex !== self) throw ex;