Why do I get different results during ftp communication when using ls filename and ls./filename

Question

I am connecting to a proftd server as client and try to list files. I use ls command with two different parameters:

1. ls filename

   ftp> ls test.txt  
   229 Entering Extended Passive Mode (|||41257|)  
   150 Opening ASCII mode data connection for file list  
   -rw-r-----   1 myftpuser (?)          9939 Jul 13 09:38 test.txt  
   226 Transfer complete.  
   ftp> 
   

2. ls ./filename

   ftp> ls ./test.txt  
   229 Entering Extended Passive Mode (|||41257|)  
   150 Opening ASCII mode data connection for file list  
   226 Transfer complete.  
   ftp> 
   

I am in the right directory in both case.
Why do I get 2 different result?

Answer 1

Turns out this is a real bug in ProFTPD; see Bug#4259.