diff --git a/changelogs/unreleased/jc-fix-gitlab-config-legacy-config.yml b/changelogs/unreleased/jc-fix-gitlab-config-legacy-config.yml
new file mode 100644
index 0000000000000000000000000000000000000000..e2550140867aaaf33d3bb6838f8859f7378b0b62
--- /dev/null
+++ b/changelogs/unreleased/jc-fix-gitlab-config-legacy-config.yml
@@ -0,0 +1,5 @@
+---
+title: Use guard in fetch_legacy_config
+merge_request: 1345
+author:
+type: fixed
diff --git a/ruby/gitlab-shell/lib/gitlab_config.rb b/ruby/gitlab-shell/lib/gitlab_config.rb
index 086ef2fad01be98a519bdfe22abb136d9dbafebe..19f03c346d58e5bbebf3fb5a0226392a93f5573f 100644
--- a/ruby/gitlab-shell/lib/gitlab_config.rb
+++ b/ruby/gitlab-shell/lib/gitlab_config.rb
@@ -56,7 +56,7 @@ class GitlabConfig
   end
 
   def fetch_from_legacy_config(key, default)
-    legacy_config.fetch(key, default)
+    legacy_config[key] || default
   end
 
   private
diff --git a/ruby/gitlab-shell/spec/gitlab_config_spec.rb b/ruby/gitlab-shell/spec/gitlab_config_spec.rb
index af6f1358f5a57c564b5dbc0949ce60297566907a..6de0cd08e2148b7b67b2ab6a195c9147d74f28de 100644
--- a/ruby/gitlab-shell/spec/gitlab_config_spec.rb
+++ b/ruby/gitlab-shell/spec/gitlab_config_spec.rb
@@ -31,4 +31,25 @@ describe GitlabConfig do
       is_expected.to eq('text')
     end
   end
+
+  describe '#fetch_from_legacy_config' do
+    let(:key) { 'yaml_key' }
+
+    where(:yaml_value, :default, :expected_value) do
+      [
+        ['a', 'b', 'a'],
+        [nil, 'b', 'b'],
+        ['a', nil, 'a'],
+        [nil, {}, {}]
+      ]
+    end
+
+    with_them do
+      it 'returns the correct value' do
+        config_data[key] = yaml_value
+
+        expect(config.fetch_from_legacy_config(key, default)).to eq(expected_value)
+      end
+    end
+  end
 end