diff --git a/src/plans.js b/src/plans.js
index 5d7aad1..b329db2 100644
--- a/src/plans.js
+++ b/src/plans.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const { pick, uid, validateKey, assert } = require('./util')
+const { pick, uid, validateKey, assert, assertMetadata } = require('./util')
 
 const PLAN_PREFIX = 'plan_'
 const PLAN_QUOTA_PERIODS = ['day', 'week', 'month']
@@ -22,7 +22,7 @@ module.exports = ({ serialize, deserialize, redis } = {}) => {
    * @param {number} [options.throttle.rateLimit] - The rate limit of the plan.
    * @param {Object} [options.metadata] - Any extra information can be attached here.
    *
-   * @returns {Object|null} The plan object, null if it doesn't exist.
+   * @returns {Object} The plan object.
    */
   const create = async (opts = {}) => {
     assert(typeof opts.name === 'string' && opts.name.length > 0, 'The argument `name` is required.')
@@ -31,6 +31,7 @@ module.exports = ({ serialize, deserialize, redis } = {}) => {
       `The argument \`quota.period\` must be ${PLAN_QUOTA_PERIODS.map(period => `\`${period}\``).join(' or ')}.`
     )
     assert(opts.quota.limit > 0, 'The argument `quota.limit` must be a positive number.')
+    opts.metadata = assertMetadata(opts.metadata)
     const plan = pick(opts, PLAN_FIELDS.concat(PLAN_FIELDS_OBJECT))
     plan.id = await uid({ redis, prefix: PLAN_PREFIX, size: 5 })
     plan.createdAt = plan.updatedAt = Date.now()
@@ -88,7 +89,7 @@ module.exports = ({ serialize, deserialize, redis } = {}) => {
   const update = async (planId, opts) => {
     const currentPlan = await retrieve(planId, { throwError: true })
     const quota = Object.assign(currentPlan.quota, opts.quota)
-    const metadata = Object.assign({}, currentPlan.metadata, opts.metadata)
+    const metadata = Object.assign({}, currentPlan.metadata, assertMetadata(opts.metadata))
     const plan = Object.assign(currentPlan, pick(opts, PLAN_FIELDS), {
       quota,
       updatedAt: Date.now()
diff --git a/src/util.js b/src/util.js
index 9ae601e..ea0b730 100644
--- a/src/util.js
+++ b/src/util.js
@@ -36,9 +36,30 @@ const validateKey =
       return id
     }
 
+const assertMetadata = metadata => {
+  if (metadata) {
+    assert(isPlainObject(metadata), 'The metadata must be a flat object.')
+    Object.keys(metadata).forEach(key => {
+      assert(!isPlainObject(metadata[key]), `The metadata field '${key}' can't be an object.`)
+      if (metadata[key] === undefined) delete metadata[key]
+    })
+    return Object.keys(metadata).length ? metadata : undefined
+  }
+}
+
+const isPlainObject = value => {
+  if (!value || typeof value !== 'object' || value.toString() !== '[object Object]') {
+    return false
+  }
+
+  const prototype = Object.getPrototypeOf(value)
+  return prototype === null || prototype === Object.prototype
+}
+
 module.exports = {
-  uid,
-  pick,
   assert,
+  assertMetadata,
+  pick,
+  uid,
   validateKey
 }
diff --git a/test/plans.js b/test/plans.js
index c62a359..f84b343 100644
--- a/test/plans.js
+++ b/test/plans.js
@@ -50,6 +50,51 @@ test('.create # `quota` is required', async t => {
   }
 })
 
+test('.create # `metadata` must be a flat object', async t => {
+  {
+    const error = await t.throwsAsync(
+      plans.create({
+        name: 'free tier',
+        quota: { period: 'week', limit: 1000 },
+        metadata: { tier: { type: 'new' } }
+      })
+    )
+    t.is(error.message, "The metadata field 'tier' can't be an object.")
+    t.is(error.name, 'TypeError')
+  }
+  {
+    const error = await t.throwsAsync(
+      plans.create({
+        name: 'free tier',
+        quota: { period: 'week', limit: 1000 },
+        metadata: 'foo'
+      })
+    )
+    t.is(error.message, 'The metadata must be a flat object.')
+    t.is(error.name, 'TypeError')
+  }
+})
+
+test('.create # `metadata` as undefined is omitted', async t => {
+  {
+    const plan = await plans.create({
+      name: 'free tier',
+      quota: { period: 'week', limit: 1000 },
+      metadata: { tier: undefined }
+    })
+    t.is(plan.metadata, undefined)
+  }
+  {
+    const plan = await plans.create({
+      name: 'free tier',
+      quota: { period: 'week', limit: 1000 },
+      metadata: { tier: 'free', version: undefined }
+    })
+
+    t.deepEqual(Object.keys(plan.metadata), ['tier'])
+  }
+})
+
 test('.create', async t => {
   const plan = await plans.create({
     name: 'free tier',
@@ -68,7 +113,11 @@ test('.create', async t => {
   t.deepEqual(plan.throttle, { burstLimit: 1000, rateLimit: 10 })
 })
 
-test('.retrieve', async t => {
+test('.retrieve # a plan not previosuly declared', async t => {
+  t.is(await plans.retrieve('plan_1'), null)
+})
+
+test('.retrieve # a plan previosuly declared', async t => {
   const { id } = await plans.create({
     name: 'free tier',
     quota: { limit: 3000, period: 'day' }
@@ -108,13 +157,59 @@ test('.update', async t => {
 })
 
 test('.update # add metadata', async t => {
+  {
+    const { id } = await plans.create({
+      name: 'free tier',
+      quota: { limit: 3000, period: 'day' }
+    })
+
+    const plan = await plans.update(id, { metadata: { tier: 'free' } })
+    t.is(plan.metadata.tier, 'free')
+  }
+  {
+    const { id } = await plans.create({
+      name: 'free tier',
+      quota: { limit: 3000, period: 'day' }
+    })
+
+    await plans.update(id, { metadata: { tier: 'free' } })
+    const plan = await plans.update(id, { metadata: { tier: 'free', version: 2 } })
+    t.is(plan.metadata.tier, 'free')
+    t.is(plan.metadata.version, 2)
+  }
+})
+
+test('.update # metadata must be a flat object', async t => {
   const { id } = await plans.create({
     name: 'free tier',
     quota: { limit: 3000, period: 'day' }
   })
 
-  const plan = await plans.update(id, { metadata: { tier: 'free' } })
-  t.is(plan.metadata.tier, 'free')
+  const error = await t.throwsAsync(plans.update(id, { metadata: { tier: { type: 'new' } } }))
+  t.is(error.message, "The metadata field 'tier' can't be an object.")
+  t.is(error.name, 'TypeError')
+})
+
+test('.update # metadata as undefined is omitted', async t => {
+  {
+    const { id } = await plans.create({
+      name: 'free tier',
+      quota: { limit: 3000, period: 'day' }
+    })
+
+    const plan = await plans.update(id, { metadata: { tier: undefined } })
+    t.is(plan.metadata, undefined)
+  }
+
+  {
+    const { id } = await plans.create({
+      name: 'free tier',
+      quota: { limit: 3000, period: 'day' }
+    })
+
+    const plan = await plans.update(id, { metadata: { tier: 'free', version: undefined } })
+    t.deepEqual(Object.keys(plan.metadata), ['tier'])
+  }
 })
 
 test('.update # prevent to add random data', async t => {
@@ -126,6 +221,17 @@ test('.update # prevent to add random data', async t => {
   t.is(plan.foo, undefined)
 })
 
+test('.update # prevent to modify the plan id', async t => {
+  const { id } = await plans.create({
+    name: 'free tier',
+    quota: { limit: 3000, period: 'day' }
+  })
+
+  const plan = await plans.update(id, { id: 'foo' })
+
+  t.is(plan.id, id)
+})
+
 test('.update # error if plan does not exist', async t => {
   {
     const error = await t.throwsAsync(plans.update('id', { foo: 'bar' }))