diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 80d1bba..2b06f15 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9072,6 +9072,7 @@
             int decimal = Py_UNICODE_TODECIMAL(ch);
             if (decimal < 0) {
                 out[i] = '?';
+                out[i+1] = '\0';
                 _PyUnicode_LENGTH(result) = i + 1;
                 break;
             }
@@ -9079,6 +9080,7 @@
         }
     }
 
+    assert(_PyUnicode_CheckConsistency(result, 1));
     return result;
 }