diff --git a/src/error.js b/src/error.js
index 82db93f..b921d7c 100644
--- a/src/error.js
+++ b/src/error.js
@@ -1,13 +1,6 @@
 'use strict'
 
-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
-}
+const { isPlainObject } = require('./util')
 
 class OpenKeyError extends Error {
   constructor (props) {
@@ -29,15 +22,15 @@ const errors = [
   ['METADATA_NOT_FLAT_OBJECT', () => 'The metadata must be a flat object.'],
   ['METADATA_INVALID', key => `The metadata field '${key}' can't be an object.`]
 ].reduce((acc, [code, message]) => {
-  acc[code] = (...input) => new OpenKeyError({ code, message: message(...input) })
+  acc[code] = args => new OpenKeyError({ code, message: message.apply(null, args()) })
   return acc
 }, {})
 
-const assert = (condition, code, ...opts) => {
+const assert = (condition, code, args = () => []) => {
   return (
     condition ||
     (() => {
-      throw errors[code].apply(null, opts)
+      throw errors[code](args)
     })()
   )
 }
@@ -46,7 +39,7 @@ const assertMetadata = metadata => {
   if (metadata) {
     assert(isPlainObject(metadata), 'METADATA_NOT_FLAT_OBJECT')
     Object.keys(metadata).forEach(key => {
-      assert(!isPlainObject(metadata[key]), 'METADATA_INVALID', key)
+      assert(!isPlainObject(metadata[key]), 'METADATA_INVALID', () => [key])
       if (metadata[key] === undefined) delete metadata[key]
     })
     return Object.keys(metadata).length ? metadata : undefined
diff --git a/src/keys.js b/src/keys.js
index 584c989..c3e752a 100644
--- a/src/keys.js
+++ b/src/keys.js
@@ -41,7 +41,7 @@ module.exports = ({ serialize, deserialize, plans, redis, prefix } = {}) => {
    */
   const retrieve = async (value, { throwError = false } = {}) => {
     const key = await redis.get(prefixKey(value))
-    if (throwError) assert(key !== null, 'KEY_NOT_EXIST', value)
+    if (throwError) assert(key !== null, 'KEY_NOT_EXIST', () => [value])
     else if (key === null) return null
     return Object.assign({ value }, deserialize(key))
   }
@@ -55,7 +55,7 @@ module.exports = ({ serialize, deserialize, plans, redis, prefix } = {}) => {
    */
   const del = async value => {
     const isDeleted = (await redis.del(prefixKey(value))) === 1
-    assert(isDeleted, 'KEY_NOT_EXIST', value)
+    assert(isDeleted, 'KEY_NOT_EXIST', () => [value])
     return isDeleted
   }
 
diff --git a/src/plans.js b/src/plans.js
index 1f05e66..d5e9795 100644
--- a/src/plans.js
+++ b/src/plans.js
@@ -31,7 +31,7 @@ module.exports = ({ serialize, deserialize, redis, keys, prefix } = {}) => {
     if (metadata) plan.metadata = metadata
     plan.createdAt = plan.updatedAt = Date.now()
     const isCreated = (await redis.set(prefixKey(opts.id), serialize(plan), 'NX')) === 'OK'
-    assert(isCreated, 'PLAN_ALREADY_EXIST', opts.id)
+    assert(isCreated, 'PLAN_ALREADY_EXIST', () => [opts.id])
     return Object.assign({ id: opts.id }, plan)
   }
 
@@ -47,7 +47,7 @@ module.exports = ({ serialize, deserialize, redis, keys, prefix } = {}) => {
    */
   const retrieve = async (id, { throwError = false } = {}) => {
     const plan = await redis.get(prefixKey(id))
-    if (throwError) assert(plan !== null, 'PLAN_NOT_EXIST', id)
+    if (throwError) assert(plan !== null, 'PLAN_NOT_EXIST', () => [id])
     else if (plan === null) return null
     return Object.assign({ id }, deserialize(plan))
   }
@@ -63,10 +63,9 @@ module.exports = ({ serialize, deserialize, redis, keys, prefix } = {}) => {
   const del = async id => {
     const allKeys = await keys().list()
     const key = allKeys.find(key => key.plan === id)
-    // TODO: try to improve this
-    assert(key === undefined, 'KEY_IS_ASSOCIATED', id, key?.value)
+    assert(key === undefined, 'KEY_IS_ASSOCIATED', () => [id, key.value])
     const isDeleted = (await redis.del(prefixKey(id))) === 1
-    assert(isDeleted, 'PLAN_NOT_EXIST', id)
+    assert(isDeleted, 'PLAN_NOT_EXIST', () => [id])
     return isDeleted
   }
 
diff --git a/src/stats.js b/src/stats.js
index ef0db9e..11b9402 100644
--- a/src/stats.js
+++ b/src/stats.js
@@ -3,17 +3,11 @@
 const { promisify } = require('util')
 const stream = require('stream')
 
-const { Transform } = stream
+const { formatYYYMMDDDate } = require('./util')
 
+const { Transform } = stream
 const pipeline = promisify(stream.pipeline)
 
-const formatYYYMMDDDate = (now = new Date()) => {
-  const year = now.getFullYear()
-  const month = String(now.getMonth() + 1).padStart(2, '0')
-  const day = String(now.getDate()).padStart(2, '0')
-  return `${year}-${month}-${day}`
-}
-
 /**
  * 90 days in milliseconds
  */
diff --git a/src/util.js b/src/util.js
index a33fead..ed22daa 100644
--- a/src/util.js
+++ b/src/util.js
@@ -20,26 +20,11 @@ const pick = (obj, keys) => {
   return result
 }
 
-/**
- * Assert a condition, or throw an error if the condition is falsy.
- * @param {*} value - The value to assert.
- * @param {string} message - The error message.
- */
-const assert = (value, message) =>
-  value ||
-  (() => {
-    throw new TypeError(message())
-  })()
-
-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 '${key}' can't be an object.`)
-      if (metadata[key] === undefined) delete metadata[key]
-    })
-    return Object.keys(metadata).length ? metadata : undefined
-  }
+const formatYYYMMDDDate = (now = new Date()) => {
+  const year = now.getFullYear()
+  const month = String(now.getMonth() + 1).padStart(2, '0')
+  const day = String(now.getDate()).padStart(2, '0')
+  return `${year}-${month}-${day}`
 }
 
 const isPlainObject = value => {
@@ -52,8 +37,8 @@ const isPlainObject = value => {
 }
 
 module.exports = {
-  assert,
-  assertMetadata,
+  formatYYYMMDDDate,
+  isPlainObject,
   pick,
   uid
 }