diff -r c642c597d05c Doc/c-api/unicode.rst
--- a/Doc/c-api/unicode.rst	Fri Dec 02 23:15:42 2016 +0200
+++ b/Doc/c-api/unicode.rst	Sat Dec 03 01:31:42 2016 +0200
@@ -1657,8 +1657,7 @@ They all return *NULL* or ``-1`` if an e
    ASCII-encoded strings, but the function interprets the input string as
    ISO-8859-1 if it contains non-ASCII characters.
 
-   This function returns ``-1`` upon failure, so one should call
-   :c:func:`PyErr_Occurred` to check for errors.
+   This function does not raise exceptions.
 
 
 .. c:function:: PyObject* PyUnicode_RichCompare(PyObject *left,  PyObject *right,  int op)
diff -r c642c597d05c Include/unicodeobject.h
--- a/Include/unicodeobject.h	Fri Dec 02 23:15:42 2016 +0200
+++ b/Include/unicodeobject.h	Sat Dec 03 01:31:42 2016 +0200
@@ -2055,7 +2055,7 @@ PyAPI_FUNC(int) _PyUnicode_EqualToASCIII
    equal, and greater than, respectively.  It is best to pass only
    ASCII-encoded strings, but the function interprets the input string as
    ISO-8859-1 if it contains non-ASCII characters.
-   Raise an exception and return -1 on error. */
+   This function does not raise exceptions. */
 
 PyAPI_FUNC(int) PyUnicode_CompareWithASCIIString(
     PyObject *left,
diff -r c642c597d05c Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Fri Dec 02 23:15:42 2016 +0200
+++ b/Objects/unicodeobject.c	Sat Dec 03 01:31:42 2016 +0200
@@ -10975,10 +10975,24 @@ PyUnicode_CompareWithASCIIString(PyObjec
     Py_ssize_t i;
     int kind;
     Py_UCS4 chr;
+    const unsigned char *ustr = (const unsigned char *)str;
 
     assert(_PyUnicode_CHECK(uni));
-    if (PyUnicode_READY(uni) == -1)
-        return -1;
+    if (!PyUnicode_IS_READY(uni)) {
+        const wchar_t *ws = _PyUnicode_WSTR(uni);
+        /* Compare Unicode string and source character set string */
+        for (i = 0; (chr = ws[i]) && ustr[i]; i++) {
+            if (chr != ustr[i])
+                return (chr < ustr[i]) ? -1 : 1;
+        }
+        /* This check keeps Python strings that end in '\0' from comparing equal
+         to C strings identical up to that point. */
+        if (_PyUnicode_WSTR_LENGTH(uni) != i || chr)
+            return 1; /* uni is longer */
+        if (ustr[i])
+            return -1; /* str is longer */
+        return 0;
+    }
     kind = PyUnicode_KIND(uni);
     if (kind == PyUnicode_1BYTE_KIND) {
         const void *data = PyUnicode_1BYTE_DATA(uni);