Zlib::GzipFile::Error: not in gzip format with input file codec => "gzip_lines"

I am not able to use the logstash-codec-gzip_lines plugin because it tells me that .gz files are not in gzip format, but if I check the file I find no specific issue:

gunzip -tv sl_trace.trace.gz
sl_trace.trace.gz:          OK
gunzip --version
gunzip (gzip) 1.5
Copyright (C) 2007, 2011-2012 Free Software Foundation, Inc.
This is free software.  You may redistribute copies of it under the terms of
the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
There is NO WARRANTY, to the extent permitted by law.

Written by Paul Eggert.

With the following config:

   file {
     path => "/tmp/traces/sl_*.gz"
     codec => "gzip_lines"
     type => "sl"