makepkg: ERROR: One or more PGP signatures could not be verified!

$ makepkg -Acs
==> Making package: cower 17-2 (Tue 19 Sep 09:58:38 JST 2017)
==> Checking runtime dependencies...
==> Checking buildtime dependencies...
==> Retrieving sources...
  -> Found cower-17.tar.gz
  -> Found cower-17.tar.gz.sig
==> Validating source files with md5sums...
    cower-17.tar.gz ... Passed
    cower-17.tar.gz.sig ... Skipped
==> Verifying source file signatures with gpg...
    cower-17.tar.gz ... FAILED (unknown public key 1EB2638FF56C0C53)
==> ERROR: One or more PGP signatures could not be verified!

pub   rsa2048 2011-06-25 [SC]
         487EACC08557AD082088DABA1EB2638FF56C0C53
uid           [  full  ] Dave Reisner <d@falconindy.com>
uid           [  full  ] Dave Reisner <dreisner@archlinux.org>
sub   rsa2048 2011-06-25 [E]

pub   rsa2048 2011-06-25 [SC]
      487EACC08557AD082088DABA1EB2638FF56C0C53
uid           [ unknown] Dave Reisner <d@falconindy.com>
uid           [ unknown] Dave Reisner <dreisner@archlinux.org>
sub   rsa2048 2011-06-25 [E]