Checks for data type before expansion

Expand variable only if its of the type String

lib/gitlab/ci/config/external/mapper.rb

@@ -105,8 +105,19 @@ module Gitlab
             if data.is_a?(String)
               expand(data)
             else
-              data.transform_values do |values|
-                values.is_a?(Array) ? values.map { |value| expand(value) } : expand(values)
+              transform(data)
+            end
+          end
+
+          def transform(data)
+            data.transform_values do |values|
+              case values
+              when Array
+                values.map { |value| expand(value.to_s) }
+              when String
+                expand(values)
+              else
+                values
               end
             end
           end

spec/lib/gitlab/ci/config/external/mapper_spec.rb

@@ -322,6 +322,19 @@ RSpec.describe Gitlab::Ci::Config::External::Mapper do
         end
       end
 
+      context 'when include variable has an unsupported type for variable expansion' do
+        let(:values) do
+          { include: { project: project.id, file: local_file },
+            image: 'ruby:2.7' }
+        end
+
+        it 'does not invoke expansion for the variable', :aggregate_failures do
+          expect(ExpandVariables).not_to receive(:expand).with(project.id, context_params[:variables])
+
+          expect { subject }.to raise_error(described_class::AmbigiousSpecificationError)
+        end
+      end
+
       context 'when feature flag is turned off' do
         let(:values) do
           { include: full_local_file_path }