diff --git a/index.js b/index.js
index ec6a8a5..ecf8371 100644
--- a/index.js
+++ b/index.js
@@ -7,10 +7,12 @@
 
 'use strict';
 
+const { deleteProperty } = Reflect;
+const isPrimitive = require('is-primitive');
 const isPlainObject = require('is-plain-object');
 
-const isObject = val => {
-  return (typeof val === 'object' && val !== null) || typeof val === 'function';
+const isObject = value => {
+  return (typeof value === 'object' && value !== null) || typeof value === 'function';
 };
 
 const isUnsafeKey = key => {
@@ -18,8 +20,8 @@ const isUnsafeKey = key => {
 };
 
 const validateKey = key => {
-  if (typeof key !== 'string' && typeof key !== 'number') {
-    key = String(key);
+  if (!isPrimitive(key)) {
+    throw new TypeError('Object keys must be strings or symbols');
   }
 
   if (isUnsafeKey(key)) {
@@ -27,7 +29,7 @@ const validateKey = key => {
   }
 };
 
-const toString = input => {
+const toStringKey = input => {
   return Array.isArray(input) ? input.flat().map(String).join(',') : input;
 };
 
@@ -43,71 +45,74 @@ const createMemoKey = (input, options) => {
 };
 
 const memoize = (input, options, fn) => {
-  const key = toString(options ? createMemoKey(input, options) : input);
+  const key = toStringKey(options ? createMemoKey(input, options) : input);
   validateKey(key);
 
-  const val = setValue.cache.get(key) || fn();
-  setValue.cache.set(key, val);
-  return val;
+  const value = setValue.cache.get(key) || fn();
+  setValue.cache.set(key, value);
+  return value;
 };
 
-const isNumber = value => {
-  if (value.trim() !== '') {
-    const number = Number(value);
-    return { is: Number.isInteger(number), number };
-  }
-  return { is: false };
-};
-
-const splitString = (input, options) => {
-  const opts = options || {};
-  const sep = opts.separator || '.';
-  const preserve = sep === '/' ? false : opts.preservePaths;
+const splitString = (input, options = {}) => {
+  const sep = options.separator || '.';
+  const preserve = sep === '/' ? false : options.preservePaths;
 
-  if (typeof input === 'symbol') {
+  if (typeof input === 'string' && preserve !== false && /\//.test(input)) {
     return [input];
   }
 
-  if (typeof opts.split === 'function') {
-    return opts.split(input);
-  }
+  const parts = [];
+  let part = '';
 
-  const keys = Array.isArray(input) ? input : input.split(sep);
-
-  if (typeof input === 'string' && preserve !== false && /\//.test(input)) {
-    return [input];
-  }
+  const push = part => {
+    let number;
+    if (part.trim() !== '' && Number.isInteger((number = Number(part)))) {
+      parts.push(number);
+    } else {
+      parts.push(part);
+    }
+  };
 
-  for (let i = 0; i < keys.length; i++) {
-    if (typeof keys[i] !== 'string') break;
-    const { is, number } = isNumber(keys[i]);
+  for (let i = 0; i < input.length; i++) {
+    const value = input[i];
 
-    if (is) {
-      keys[i] = number;
+    if (value === '\\') {
+      part += input[++i];
       continue;
     }
 
-    while (keys[i] && i < keys.length && keys[i].endsWith('\\') && typeof keys[i + 1] === 'string') {
-      keys[i] = keys[i].slice(0, -1) + sep + keys.splice(i + 1, 1);
+    if (value === sep) {
+      push(part);
+      part = '';
+      continue;
     }
+
+    part += value;
+  }
+
+  if (part) {
+    push(part);
   }
 
-  return keys;
+  return parts;
 };
 
 const split = (input, options) => {
+  if (options && typeof options.split === 'function') return options.split(input);
+  if (typeof input === 'symbol') return [input];
+  if (Array.isArray(input)) return input;
   return memoize(input, options, () => splitString(input, options));
 };
 
-const setProp = (obj, prop, value, options) => {
+const assignProp = (obj, prop, value, options) => {
   validateKey(prop);
 
   // Delete property when "value" is undefined
   if (value === undefined) {
-    delete obj[prop];
+    deleteProperty(obj, prop);
 
   } else if (options && options.merge) {
-    const merge = options.merge === true ? Object.assign : options.merge;
+    const merge = options.merge === 'function' ? options.merge : Object.assign;
 
     // Only merge plain objects
     if (merge && isPlainObject(obj[prop]) && isPlainObject(value)) {
@@ -123,28 +128,25 @@ const setProp = (obj, prop, value, options) => {
   return obj;
 };
 
-const setValue = (obj, path, value, options) => {
-  if (!path) return obj;
-  if (!isObject(obj)) return obj;
+const setValue = (target, path, value, options) => {
+  if (!path || !isObject(target)) return target;
 
   const keys = split(path, options);
-  const len = keys.length;
-  const target = obj;
+  let obj = target;
 
-  for (let i = 0; i < len; i++) {
+  for (let i = 0; i < keys.length; i++) {
     const key = keys[i];
     const next = keys[i + 1];
 
     validateKey(key);
 
     if (next === undefined) {
-      setProp(obj, key, value, options);
+      assignProp(obj, key, value, options);
       break;
     }
 
     if (typeof next === 'number' && !Array.isArray(obj[key])) {
-      obj[key] = [];
-      obj = obj[key];
+      obj = obj[key] = [];
       continue;
     }
 
@@ -158,6 +160,7 @@ const setValue = (obj, path, value, options) => {
   return target;
 };
 
+setValue.split = split;
 setValue.cache = new Map();
 setValue.clear = () => {
   setValue.cache = new Map();
diff --git a/package.json b/package.json
index 3865c1f..14a366e 100644
--- a/package.json
+++ b/package.json
@@ -28,11 +28,12 @@
     "test": "mocha"
   },
   "dependencies": {
-    "is-plain-object": "^2.0.4"
+    "is-plain-object": "^2.0.4",
+    "is-primitive": "^3.0.1"
   },
   "devDependencies": {
     "gulp-format-md": "^2.0.0",
-    "mocha": "^8.3.2",
+    "mocha": "^9.1.1",
     "split-string": "^6.1.0"
   },
   "keywords": [