Test error: serr declared but not used

[eclipseo]

With Go 1.12.5

Testing    in: /builddir/build/BUILD/mint-83ba9bc2ead9cdf8b0c8bd94e8a41243e5f6ed88/_build/src
         PATH: /builddir/build/BUILD/mint-83ba9bc2ead9cdf8b0c8bd94e8a41243e5f6ed88/_build/bin:/builddir/.local/bin:/builddir/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin
       GOPATH: /builddir/build/BUILD/mint-83ba9bc2ead9cdf8b0c8bd94e8a41243e5f6ed88/_build:/usr/share/gocode
  GO111MODULE: off
      command: go test -buildmode pie -compiler gc -ldflags "-X github.com/bifurcation/mint/version=0 -X github.com/bifurcation/mint/version.commit=83ba9bc2ead9cdf8b0c8bd94e8a41243e5f6ed88 -extldflags '-Wl,-z,relro -Wl,--as-needed  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld '"
      testing: github.com/bifurcation/mint
github.com/bifurcation/mint
# github.com/bifurcation/mint
vet: ./tls_test.go:188:6: serr declared but not used
FAIL	github.com/bifurcation/mint [build failed]