diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 293b5c4..02aaf6e 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -11081,6 +11081,12 @@
 {
     size_t len;
     assert(_PyUnicode_CHECK(unicode));
+    assert(str);
+#ifndef NDEBUG
+    for (const char *p = str; *p; p++) {
+        assert((unsigned char)*p < 128);
+    }
+#endif
     if (PyUnicode_READY(unicode) == -1) {
         /* Memory error or bad data */
         PyErr_Clear();
@@ -11101,6 +11107,11 @@
 
     assert(_PyUnicode_CHECK(left));
     assert(right->string);
+#ifndef NDEBUG
+    for (const char *p = right->string; *p; p++) {
+        assert((unsigned char)*p < 128);
+    }
+#endif
 
     if (PyUnicode_READY(left) == -1) {
         /* memory error or bad data */