apt-get update GPG error

["Bulent Sener" <bsener@inonu.edu.tr>]

Merhabalar,

fbe-buro:/home/bsener# apt-get update
Get:1 ftp://ftp.tr.debian.org unstable Release.gpg [197B]
Get:2 ftp://ftp.tr.debian.org unstable Release [34.1kB]
Ign ftp://ftp.tr.debian.org unstable Release
Get:3 ftp://ftp.tr.debian.org unstable/main Packages [3519kB]
Get:4 ftp://ftp.tr.debian.org unstable/contrib Packages [72.8kB]
Get:5 ftp://ftp.tr.debian.org unstable/non-free Packages [66.1kB]
Fetched 3692kB in 9s (400kB/s)
Reading Package Lists... Done
W: GPG error: ftp://ftp.tr.debian.org unstable Release: The following signatures
couldn't be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D
W: You may want to run apt-get update to correct these problems

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Bülent ŞENER