diff --git a/ext/java/org/jruby/ext/stringio/StringIO.java b/ext/java/org/jruby/ext/stringio/StringIO.java
index 2bd9ab6..227b30a 100644
--- a/ext/java/org/jruby/ext/stringio/StringIO.java
+++ b/ext/java/org/jruby/ext/stringio/StringIO.java
@@ -1559,6 +1559,7 @@ private void ungetbyteCommon(ThreadContext context, byte[] ungetBytes, int cp, i
                     strBytelist = str.getByteList();
                     strBytelist.setRealSize(len + rest);
                 } else {
+                    str.modify();
                     strBytelist = str.getByteList();
                 }
                 strBytes = strBytelist.unsafeBytes();