diff --git a/lib/kamal/cli/build.rb b/lib/kamal/cli/build.rb
index 9ce1f4a62..c655c6aeb 100644
--- a/lib/kamal/cli/build.rb
+++ b/lib/kamal/cli/build.rb
@@ -140,7 +140,7 @@ def mirror_hosts
         mirror_hosts = Concurrent::Hash.new
         on(KAMAL.hosts) do |host|
           first_mirror = capture_with_info(*KAMAL.builder.first_mirror).strip.presence
-          mirror_hosts[first_mirror] ||= host if first_mirror
+          mirror_hosts[first_mirror] ||= host.to_s if first_mirror
         rescue SSHKit::Command::Failed => e
           raise unless e.message =~ /error calling index: reflect: slice index out of range/
         end
diff --git a/test/cli/build_test.rb b/test/cli/build_test.rb
index 4035ea511..18ff254b4 100644
--- a/test/cli/build_test.rb
+++ b/test/cli/build_test.rb
@@ -185,7 +185,7 @@ class CliBuildTest < CliTestCase
     run_command("pull").tap do |output|
       assert_match /Pulling image on 1\.1\.1\.\d to seed the mirror\.\.\./, output
       assert_match "Pulling image on remaining hosts...", output
-      assert_match /docker pull dhh\/app:999/, output
+      assert_equal 4, output.scan(/docker pull dhh\/app:999/).size, output
       assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the 'service' label\" && exit 1)", output
     end
   end
@@ -199,7 +199,7 @@ class CliBuildTest < CliTestCase
     run_command("pull").tap do |output|
       assert_match /Pulling image on 1\.1\.1\.\d, 1\.1\.1\.\d to seed the mirrors\.\.\./, output
       assert_match "Pulling image on remaining hosts...", output
-      assert_match /docker pull dhh\/app:999/, output
+      assert_equal 4, output.scan(/docker pull dhh\/app:999/).size, output
       assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the 'service' label\" && exit 1)", output
     end
   end