diff --git a/Doc/c-api/arg.rst b/Doc/c-api/arg.rst
index f34e4b4..4774861 100644
--- a/Doc/c-api/arg.rst
+++ b/Doc/c-api/arg.rst
@@ -136,7 +136,7 @@
    In both cases, *\*buffer_length* is set to the length of the encoded data
    without the trailing NUL byte.
 
-``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
+``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer, int \*buffer_length]
    Same as ``es#`` except that string objects are passed through without
    recoding them. Instead, the implementation assumes that the string object
    uses the encoding passed in as parameter.