diff --git a/Include/pyctype.h b/Include/pyctype.h
index 0d211ec799c..c5cc61431d0 100644
--- a/Include/pyctype.h
+++ b/Include/pyctype.h
@@ -11,6 +11,9 @@
 
 extern const unsigned int _Py_ctype_table[256];
 
+/* Unlike their C counterparts, the following macros are not meant to
+ * handle an int with any of the values [EOF, 0-UCHAR_MAX]. The argument
+ * must be a signed/unsigned char. */
 #define Py_ISLOWER(c)  (_Py_ctype_table[Py_CHARMASK(c)] & PY_CTF_LOWER)
 #define Py_ISUPPER(c)  (_Py_ctype_table[Py_CHARMASK(c)] & PY_CTF_UPPER)
 #define Py_ISALPHA(c)  (_Py_ctype_table[Py_CHARMASK(c)] & PY_CTF_ALPHA)