diff -r ba14f8b61bd8 Include/unicodeobject.h
--- a/Include/unicodeobject.h	Wed Nov 16 16:13:13 2016 +0200
+++ b/Include/unicodeobject.h	Wed Nov 16 18:07:31 2016 +0200
@@ -2038,7 +2038,7 @@ PyAPI_FUNC(int) PyUnicode_Compare(
 
 #ifndef Py_LIMITED_API
 /* Test whether a unicode is equal to ASCII identifier.  Return 1 if true,
-   0 otherwise.  Return 0 if any argument contains non-ASCII characters.
+   0 otherwise.  The right argument must be ASCII identifier.
    Any error occurs inside will be cleared before return. */
 
 PyAPI_FUNC(int) _PyUnicode_EqualToASCIIId(
@@ -2060,7 +2060,7 @@ PyAPI_FUNC(int) PyUnicode_CompareWithASC
 
 #ifndef Py_LIMITED_API
 /* Test whether a unicode is equal to ASCII string.  Return 1 if true,
-   0 otherwise.  Return 0 if any argument contains non-ASCII characters.
+   0 otherwise.  The right argument must be ASCII-encoded string.
    Any error occurs inside will be cleared before return. */
 
 PyAPI_FUNC(int) _PyUnicode_EqualToASCIIString(
diff -r ba14f8b61bd8 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Wed Nov 16 16:13:13 2016 +0200
+++ b/Objects/unicodeobject.c	Wed Nov 16 18:07:31 2016 +0200
@@ -11062,6 +11062,12 @@ int
 {
     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();
@@ -11082,6 +11088,11 @@ int
 
     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 */