diff --git a/auth.js b/auth.js
index 298f7b6..607fb26 100644
--- a/auth.js
+++ b/auth.js
@@ -8,11 +8,13 @@ const DEFAULT_RELATION = 'or'
 function fastifyAuth (fastify, opts, next) {
   if (opts.defaultRelation && opts.defaultRelation !== 'or' && opts.defaultRelation !== 'and') {
     return next(new Error("The value of default relation should be one of ['or', 'and']"))
-  } else if (!opts.defaultRelation) {
-    opts.defaultRelation = DEFAULT_RELATION
   }
 
-  fastify.decorate('auth', auth(opts))
+  const pluginOptions = {
+    defaultRelation: opts.defaultRelation || DEFAULT_RELATION
+  }
+
+  fastify.decorate('auth', auth(pluginOptions))
   next()
 }
 
diff --git a/test/auth.test.js b/test/auth.test.js
index d0d63b5..867b61c 100644
--- a/test/auth.test.js
+++ b/test/auth.test.js
@@ -17,6 +17,20 @@ test('registering plugin with invalid default relation', (t, done) => {
   })
 })
 
+test('registering plugin should not mutate opts', (t, done) => {
+  t.plan(2)
+
+  const fastify = Fastify()
+  const opts = {}
+  fastify.register(fastifyAuth, opts)
+
+  fastify.ready((err) => {
+    t.assert.ifError(err)
+    t.assert.strictEqual(opts.defaultRelation, undefined)
+    done()
+  })
+})
+
 test('Clean status code through auth pipeline', (t, done) => {
   t.plan(3)