diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index c40163f..a4536b1 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -131,6 +131,10 @@
         return -1;
 
     assert(PyUnicode_Check(decoded));
+    if (PyUnicode_READY(decoded)) {
+        Py_DECREF(decoded);
+        return -1;
+    }
     len = PyUnicode_GET_LENGTH(decoded);
 
     assert(len >= 0);