diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 130ca48..d80ff71 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8170,6 +8170,7 @@
            size_t buflen,
            PyObject *v)
 {
+    PyObject *s;
     /* presume that the buffer is at least 2 characters long */
     if (PyUnicode_Check(v)) {
         if (PyUnicode_GET_SIZE(v) != 1)
@@ -8180,7 +8181,14 @@
     else if (PyString_Check(v)) {
         if (PyString_GET_SIZE(v) != 1)
             goto onError;
-        buf[0] = (Py_UNICODE)PyString_AS_STRING(v)[0];
+        /* #7649: if the char is a non-ascii (i.e. in range(0x80,0x100)) byte
+           string, "u'%c' % char" should fail with a UnicodeDecodeError */
+        s = PyUnicode_FromStringAndSize(PyString_AS_STRING(v), 1);
+        /* if the char is not decodable return -1 */
+        if (s == NULL)
+            return -1;
+        buf[0] = PyUnicode_AS_UNICODE(s)[0];
+        Py_DECREF(s);
     }
 
     else {