diff --git a/changelogs/unreleased/bvl-fix-500-on-fork-without-restricted-visibility-levels.yml b/changelogs/unreleased/bvl-fix-500-on-fork-without-restricted-visibility-levels.yml
new file mode 100644
index 00000000000..378f0ef7ce9
--- /dev/null
+++ b/changelogs/unreleased/bvl-fix-500-on-fork-without-restricted-visibility-levels.yml
@@ -0,0 +1,5 @@
+---
+title: Fix forking projects when no restricted visibility levels are defined applicationwide
+merge_request: 16881
+author:
+type: fixed
diff --git a/lib/gitlab/visibility_level.rb b/lib/gitlab/visibility_level.rb
index 6ced06a863d..0b757b2a646 100644
--- a/lib/gitlab/visibility_level.rb
+++ b/lib/gitlab/visibility_level.rb
@@ -60,7 +60,7 @@ module Gitlab
       def allowed_levels
         restricted_levels = current_application_settings.restricted_visibility_levels
 
-        self.values - restricted_levels
+        self.values - Array(restricted_levels)
       end
 
       def closest_allowed_level(target_level)
diff --git a/spec/lib/gitlab/visibility_level_spec.rb b/spec/lib/gitlab/visibility_level_spec.rb
index d85dac630b4..2c1146ceff5 100644
--- a/spec/lib/gitlab/visibility_level_spec.rb
+++ b/spec/lib/gitlab/visibility_level_spec.rb
@@ -57,6 +57,15 @@ describe Gitlab::VisibilityLevel do
       expect(described_class.allowed_levels)
         .to contain_exactly(described_class::PRIVATE, described_class::PUBLIC)
     end
+
+    it 'returns all levels when no visibility level was set' do
+      allow(described_class)
+        .to receive_message_chain('current_application_settings.restricted_visibility_levels')
+              .and_return(nil)
+
+      expect(described_class.allowed_levels)
+        .to contain_exactly(described_class::PRIVATE, described_class::INTERNAL, described_class::PUBLIC)
+    end
   end
 
   describe '.closest_allowed_level' do