diff --git a/taglib/toolkit/tstring.cpp b/taglib/toolkit/tstring.cpp
index cd274367..f209d726 100644
--- a/taglib/toolkit/tstring.cpp
+++ b/taglib/toolkit/tstring.cpp
@@ -510,6 +510,8 @@ String String::number(int n) // static
 
 TagLib::wchar &String::operator[](int i)
 {
+  detach();
+
   return d->data[i];
 }