diff --git a/lib/vagrant/action/builtin/box_add.rb b/lib/vagrant/action/builtin/box_add.rb
index 9e8374920..f04b8b861 100644
--- a/lib/vagrant/action/builtin/box_add.rb
+++ b/lib/vagrant/action/builtin/box_add.rb
@@ -329,6 +329,17 @@ module Vagrant
             provider_url = authed_urls[0]
           end
 
+          # The architecture name used when adding the box should be
+          # the value extracted from the metadata provider
+          arch_name = metadata_provider.architecture
+
+          # In the special case where the architecture name is "unknown" and
+          # it is listed as the default architecture, unset the architecture
+          # name so it is installed without architecture information
+          if arch_name == "unknown" && metadata_provider.default_architecture
+            arch_name = nil
+          end
+
           box_add(
             [[provider_url, metadata_provider.url]],
             metadata.name,
@@ -338,7 +349,7 @@ module Vagrant
             env,
             checksum: metadata_provider.checksum,
             checksum_type: metadata_provider.checksum_type,
-            architecture: metadata_provider.architecture,
+            architecture: arch_name,
           )
         end
 
diff --git a/test/unit/vagrant/action/builtin/box_add_test.rb b/test/unit/vagrant/action/builtin/box_add_test.rb
index c3366d504..704e7626e 100644
--- a/test/unit/vagrant/action/builtin/box_add_test.rb
+++ b/test/unit/vagrant/action/builtin/box_add_test.rb
@@ -1033,7 +1033,7 @@ describe Vagrant::Action::Builtin::BoxAdd, :skip_windows, :bsdtar do
           subject.call(env)
         end
 
-        it "adds the latest version of a box with only one provider and no unknown architecture set as default" do
+        it "adds the latest version of a box with only one provider and unknown architecture set as default" do
           box_path = iso_env.box2_file(:virtualbox)
           tf = Tempfile.new(["vagrant-box-latest-version", ".json"]).tap do |f|
             f.write(
@@ -1072,7 +1072,7 @@ describe Vagrant::Action::Builtin::BoxAdd, :skip_windows, :bsdtar do
             expect(name).to eq("foo/bar")
             expect(version).to eq("0.7")
             expect(opts[:metadata_url]).to eq("file://#{tf.path}")
-            expect(opts[:architecture]).to eq("unknown")
+            expect(opts[:architecture]).to be_nil
             true
           }.and_return(box)