diff --git a/app/transformers/securityDefinitions.js b/app/transformers/securityDefinitions.js
index 0584941..df28836 100644
--- a/app/transformers/securityDefinitions.js
+++ b/app/transformers/securityDefinitions.js
@@ -27,8 +27,15 @@ module.exports = securityDefinitions => {
             + `${securityDefinitions[type][value][scope].replace(/[\r\n]/g, ' ')}|`);
         });
       } else if (value !== 'type' && securityDefinitions[type][value].replace) {
-        res.push(`|${nameResolver[value]}|`
-          + `${securityDefinitions[type][value].replace(/[\r\n]/g, ' ')}|`);
+        let key = nameResolver[value];
+        if (key === undefined) {
+          if (value.match(/^x-/i)) {
+            key = value;
+          } else {
+            return;
+          }
+        }
+        res.push(`|${key}|${securityDefinitions[type][value].replace(/[\r\n]/g, ' ')}|`);
       }
     });
     res.push('');
diff --git a/tests/transformers/securityDefinitions.spec.js b/tests/transformers/securityDefinitions.spec.js
index fa30b5a..ad0284a 100644
--- a/tests/transformers/securityDefinitions.spec.js
+++ b/tests/transformers/securityDefinitions.spec.js
@@ -42,4 +42,17 @@ describe('Security definitions', () => {
     const res = transformSecurituDefinitions(fixture);
     expect(res).to.exist;
   });
+  it('Should ignore undefined keys unless it is prefixed with x-', () => {
+    const fixture = {
+      'complex-structure': {
+        type: 'apiKey',
+        name: 'Name',
+        'x-special-key': 'Special key',
+        'unknown-key': 'Uknown key',
+      }
+    };
+    const result = transformSecurituDefinitions(fixture);
+    expect(result.match(/undefined/ig)).to.be.null;
+    expect(result.match(/x-special-key/ig)).to.be.not.null;
+  });
 });