diff --git a/src/org/mozilla/javascript/ConsString.java b/src/org/mozilla/javascript/ConsString.java
index d90e47be77..4513fbe899 100644
--- a/src/org/mozilla/javascript/ConsString.java
+++ b/src/org/mozilla/javascript/ConsString.java
@@ -32,6 +32,12 @@ public class ConsString implements CharSequence, Serializable {
     private boolean isFlat;
 
     public ConsString(CharSequence str1, CharSequence str2) {
+        if (!(str1 instanceof String) && !(str1 instanceof ConsString)) {
+            str1 = str1.toString();
+        }
+        if (!(str2 instanceof String) && !(str2 instanceof ConsString)) {
+            str2 = str2.toString();
+        }
         left = str1;
         right = str2;
         length = left.length() + right.length();
@@ -69,7 +75,7 @@ private synchronized String flatten() {
                     }
                 }
 
-                final String str = next.toString();
+                final String str = (String) next;
                 charPos -= str.length();
                 str.getChars(0, str.length(), chars, charPos);
                 next = stack.isEmpty() ? null : stack.removeFirst();