[Bug #991883] Document calling condition
diff --git a/Doc/api/abstract.tex b/Doc/api/abstract.tex
index 50a5c59..996d0ea 100644
--- a/Doc/api/abstract.tex
+++ b/Doc/api/abstract.tex
@@ -269,7 +269,8 @@
\moreargs}
Call the method named \var{method} of object \var{o} with a variable
number of C arguments. The C arguments are described by a
- \cfunction{Py_BuildValue()} format string. The format may be \NULL,
+ \cfunction{Py_BuildValue()} format string that should
+ produce a tuple. The format may be \NULL,
indicating that no arguments are provided. Returns the result of the
call on success, or \NULL{} on failure. This is the equivalent of
the Python expression \samp{\var{o}.\var{method}(\var{args})}.