diff --git a/.eslintrc b/.eslintrc
index 158f293..dd54aa6 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -19,7 +19,7 @@
   },
   "extends": ["eslint:recommended", "plugin:react/recommended", "prettier"],
   "rules": {
-    "react/no-find-dom-node": "off",
-    "react/prop-types": "off"
+    "react/prop-types": "off",
+    "no-unused-vars": ["error", { "ignoreRestSiblings": true }]
   }
 }
diff --git a/demo/index.js b/demo/index.js
index f8dbb3c..6d44689 100644
--- a/demo/index.js
+++ b/demo/index.js
@@ -117,19 +117,19 @@ class ComponentChild extends React.Component {
 class App extends React.Component {
   state = {
     arrow: false,
-    customClass: ''
+    customClass: 'hello',
   }
 
   toggleArrow = () => {
     this.setState(state => ({
-      arrow: !state.arrow
+      arrow: !state.arrow,
     }))
   }
 
-  updateCustomClass = (e) => {
-    this.setState({
-      customClass: e.target.value
-    })
+  componentDidMount() {
+    setTimeout(() => {
+      this.setState({ customClass: 'bye' })
+    }, 500)
   }
 
   render() {
@@ -174,8 +174,7 @@ class App extends React.Component {
         </Tippy>
 
         <h1>Other</h1>
-        <input type="text" placeholder="Enter class" onChange={this.updateCustomClass}/>
-        <Tippy placement="bottom" className={this.state.customClass}>
+        <Tippy trigger="click" className={this.state.customClass}>
           <button>Custom class</button>
         </Tippy>
       </main>
diff --git a/src/Tippy.js b/src/Tippy.js
index 41c88f2..e380138 100644
--- a/src/Tippy.js
+++ b/src/Tippy.js
@@ -3,7 +3,6 @@ import React, {
   cloneElement,
   useState,
   useRef,
-  useEffect,
   useLayoutEffect,
 } from 'react'
 import { createPortal } from 'react-dom'
@@ -32,20 +31,15 @@ function Tippy(props) {
     options.trigger = 'manual'
   }
 
-  useEffect(() => {
+  useLayoutEffect(() => {
     instanceRef.current = tippy(targetRef.current, options)
 
-    const { onCreate, isEnabled, isVisible, className } = props
+    const { onCreate, isEnabled, isVisible } = props
 
     if (onCreate) {
       onCreate(instanceRef.current)
     }
 
-    if (className) {
-      const { tooltip } = instanceRef.current.popperChildren
-      updateClassName(tooltip, 'add', props.className)
-    }
-
     if (isEnabled === false) {
       instanceRef.current.disable()
     }
@@ -86,15 +80,12 @@ function Tippy(props) {
   })
 
   useLayoutEffect(() => {
-    if (!isMounted) {
-      return
-    }
-
-    const { tooltip } = instanceRef.current.popperChildren
-    updateClassName(tooltip, 'add', props.className)
-
-    return () => {
-      updateClassName(tooltip, 'remove', props.className)
+    if (props.className) {
+      const { tooltip } = instanceRef.current.popperChildren
+      updateClassName(tooltip, 'add', props.className)
+      return () => {
+        updateClassName(tooltip, 'remove', props.className)
+      }
     }
   }, [props.className])
 
diff --git a/src/utils.js b/src/utils.js
index 22d8818..4e7037b 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -1,6 +1,12 @@
 export function getNativeTippyProps(props) {
-  // eslint-disable-next-line no-unused-vars
-  const { children, onCreate, isVisible, isEnabled, className, ...nativeProps } = props
+  const {
+    children,
+    onCreate,
+    isVisible,
+    isEnabled,
+    className,
+    ...nativeProps
+  } = props
   return nativeProps
 }
 
diff --git a/test/Tippy.test.js b/test/Tippy.test.js
index e95d784..e63ff82 100644
--- a/test/Tippy.test.js
+++ b/test/Tippy.test.js
@@ -52,27 +52,55 @@ describe('<Tippy />', () => {
     expect(instance.popper.querySelector('strong')).not.toBeNull()
   })
 
-  test('custom class name get added to DOM', () => {
+  test('props.className: single name is added to tooltip', () => {
     const className = 'hello'
     const { container } = render(
-      <Tippy content="tip content" className={className}>
+      <Tippy content="tooltip" className={className}>
         <button />
       </Tippy>,
     )
-    const tip = container.querySelector('button')._tippy
-    expect(tip.popper.querySelector(`.${className}`)).not.toBeNull()
+    const instance = container.querySelector('button')._tippy
+    expect(instance.popper.querySelector(`.${className}`)).not.toBeNull()
   })
 
-  test('custom class name get added to DOM', () => {
+  test('props.className: multiple names are added to tooltip', () => {
     const classNames = 'hello world'
     const { container } = render(
-      <Tippy content="tip content" className={classNames}>
+      <Tippy content="tooltip" className={classNames}>
+        <button />
+      </Tippy>,
+    )
+    const instance = container.querySelector('button')._tippy
+    expect(instance.popper.querySelector('.hello')).not.toBeNull()
+    expect(instance.popper.querySelector('.world')).not.toBeNull()
+  })
+
+  test('props.className: extra whitespace is ignored', () => {
+    const className = ' hello world  '
+    const { container } = render(
+      <Tippy content="tooltip" className={className}>
+        <button />
+      </Tippy>,
+    )
+    const { tooltip } = container.querySelector('button')._tippy.popperChildren
+    expect(tooltip.className).toBe('tippy-tooltip dark-theme hello world')
+  })
+
+  test('props.className: updating does not leave stale className behind', () => {
+    const { container, rerender } = render(
+      <Tippy content="tooltip" className="one">
+        <button />
+      </Tippy>,
+    )
+    const { tooltip } = container.querySelector('button')._tippy.popperChildren
+    expect(tooltip.classList.contains('one')).toBe(true)
+    rerender(
+      <Tippy content="tooltip" className="two">
         <button />
       </Tippy>,
     )
-    const tip = container.querySelector('button')._tippy
-    expect(tip.popper.querySelector('.hello')).not.toBeNull()
-    expect(tip.popper.querySelector('.world')).not.toBeNull()
+    expect(tooltip.classList.contains('one')).toBe(false)
+    expect(tooltip.classList.contains('two')).toBe(true)
   })
 
   test('unmount destroys the tippy instance and allows garbage collection', () => {