diff --git a/Doc/lib/libzlib.tex b/Doc/lib/libzlib.tex
index db62a270c32..a387ee9d711 100644
--- a/Doc/lib/libzlib.tex
+++ b/Doc/lib/libzlib.tex
@@ -145,7 +145,7 @@ call in order to get correct output.
 \end{memberdesc}
 
 
-\begin{methoddesc}[Decompress]{decompress}{string}{\optional{max_length}}
+\begin{methoddesc}[Decompress]{decompress}{string\optional{, max_length}}
 Decompress \var{string}, returning a string containing the
 uncompressed data corresponding to at least part of the data in
 \var{string}.  This data should be concatenated to the output produced