Question: Repo signing keys

[Hans <hans.ullrich@loop.de>]

Hi folks,

I have added a trustworthy repository and signed it with the key, But I get a 
warning: 

 aptitude update
Treffer http://apt.metasploit.com sid InRelease
Holen: 1 ftp://ftp.de.debian.org/debian stable InRelease        
Ign ftp://ftp.de.debian.org/debian stable InRelease                                          
Treffer http://security.debian.org stable/updates InRelease
Treffer ftp://ftp.de.debian.org/debian testing InRelease
Treffer ftp://ftp.de.debian.org/debian stable Release
W: http://apt.metasploit.com/dists/sid/InRelease: Signature by key 
09E55FAF4F7862CD6D558997CDFB5FA52007B954 uses weak digest algorithm (SHA1)

So, what did I do wrong? Did I sign with the wrong key? 

Thanks for any information.

Best regards

Hans