diff --git a/docs/pages/api/avatar-group.md b/docs/pages/api/avatar-group.md
index 3c18e082d6c21e..b58ca8a30319a0 100644
--- a/docs/pages/api/avatar-group.md
+++ b/docs/pages/api/avatar-group.md
@@ -26,8 +26,7 @@ You can learn more about the difference by [reading this guide](/guides/minimizi
 |:-----|:-----|:--------|:------------|
 | <span class="prop-name">children</span> | <span class="prop-type">node</span> |  | The avatars to stack. |
 | <span class="prop-name">classes</span> | <span class="prop-type">object</span> |  | Override or extend the styles applied to the component. See [CSS API](#css) below for more details. |
-| <span class="prop-name">spacing</span> | <span class="prop-type">'small'<br>&#124;&nbsp;'medium'<br>&#124;&nbsp;'large'<br>&#124;&nbsp;number</span> |  | Defines the spacing between `avatar` as `'large': -4px`, `'medium': -8px`, and `'small': -16px` or define a `number` as the spacing. |
-
+| <span class="prop-name">spacing</span> | <span class="prop-type">'large'<br>&#124;&nbsp;'medium'<br>&#124;&nbsp;'small'<br>&#124;&nbsp;number</span> | <span class="prop-default">'medium'</span> | Spacing between avatars. |
 
 The `ref` is forwarded to the root element.
 
diff --git a/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.d.ts b/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.d.ts
index 6431f8b37ffc2d..7786af1a461c09 100644
--- a/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.d.ts
+++ b/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.d.ts
@@ -15,4 +15,4 @@ export interface AvatarGroupProps
 
 export type AvatarGroupClassKey = 'root' | 'avatar';
 
-export default function AvatarGroup(props: AvatarGroupProps): JSX.Element | null;
+export default function AvatarGroup(props: AvatarGroupProps): JSX.Element;
diff --git a/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.js b/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.js
index e94524d838f5b0..eefc96f12adb20 100644
--- a/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.js
+++ b/packages/material-ui-lab/src/AvatarGroup/AvatarGroup.js
@@ -5,9 +5,8 @@ import clsx from 'clsx';
 import { withStyles } from '@material-ui/core/styles';
 
 const SPACINGS = {
-  large: -4, 
-  medium: -8, 
-  small: -16
+  large: -4,
+  small: -16,
 };
 
 export const styles = theme => ({
@@ -18,11 +17,13 @@ export const styles = theme => ({
   /* Styles applied to the avatar elements. */
   avatar: {
     border: `2px solid ${theme.palette.background.default}`,
+    marginLeft: -8,
   },
 });
 
 const AvatarGroup = React.forwardRef(function AvatarGroup(props, ref) {
-  const { children: childrenProp, classes, className, spacing, ...other } = props;
+  const { children: childrenProp, classes, className, spacing = 'medium', ...other } = props;
+
   const children = React.Children.toArray(childrenProp).filter(child => {
     if (process.env.NODE_ENV !== 'production') {
       if (isFragment(child)) {
@@ -45,7 +46,7 @@ const AvatarGroup = React.forwardRef(function AvatarGroup(props, ref) {
           className: clsx(child.props.className, classes.avatar),
           style: {
             zIndex: children.length - index,
-            marginLeft: spacing ? (SPACINGS[spacing] ? SPACINGS[spacing] : spacing) : -8,
+            marginLeft: spacing && SPACINGS[spacing] ? SPACINGS[spacing] : -spacing,
             ...child.props.style,
           },
         });
@@ -68,14 +69,14 @@ AvatarGroup.propTypes = {
    * See [CSS API](#css) below for more details.
    */
   classes: PropTypes.object,
-  /**
-   * Spacing between avatars.
-   */
-  spacing: PropTypes.oneOfType([PropTypes.oneOf(['large', 'medium', 'small']), PropTypes.number]),
   /**
    * @ignore
    */
   className: PropTypes.string,
+  /**
+   * Spacing between avatars.
+   */
+  spacing: PropTypes.oneOfType([PropTypes.oneOf(['large', 'medium', 'small']), PropTypes.number]),
 };
 
 export default withStyles(styles, { name: 'MuiAvatarGroup' })(AvatarGroup);