diff --git a/lib/openapi_parser/request_operation.rb b/lib/openapi_parser/request_operation.rb
index d0312b9..2746ce4 100644
--- a/lib/openapi_parser/request_operation.rb
+++ b/lib/openapi_parser/request_operation.rb
@@ -78,7 +78,8 @@ def initialize(status_code, response_data, headers)
     end
 
     def content_type
-      headers['Content-Type'].to_s.split(';').first.to_s
+      content_type_key = headers.keys.detect { |k| k.casecmp?('Content-Type') }
+      headers[content_type_key].to_s.split(';').first.to_s
     end
   end
 end
diff --git a/spec/openapi_parser/request_operation_spec.rb b/spec/openapi_parser/request_operation_spec.rb
index 71de301..28ca6a2 100644
--- a/spec/openapi_parser/request_operation_spec.rb
+++ b/spec/openapi_parser/request_operation_spec.rb
@@ -216,3 +216,24 @@
     end
   end
 end
+
+RSpec.describe OpenAPIParser::RequestOperation::ValidatableResponseBody do
+  describe '#content_type' do
+    context 'when key is lowercase' do
+      let(:headers) { {"content-type" => "application/json"} }
+        it 'finds the key' do
+          expect(
+            OpenAPIParser::RequestOperation::ValidatableResponseBody.new(nil, nil, headers).content_type
+          ).to eq "application/json"
+      end
+    end
+    context 'when key is mixed case' do
+      let(:headers) { {"Content-Type" => "application/json"} }
+        it 'finds the key' do
+          expect(
+            OpenAPIParser::RequestOperation::ValidatableResponseBody.new(nil, nil, headers).content_type
+          ).to eq "application/json"
+      end
+    end
+  end
+end