diff --git a/src/components/Modal.js b/src/components/Modal.js
index ee51a588..19ee8a96 100644
--- a/src/components/Modal.js
+++ b/src/components/Modal.js
@@ -42,11 +42,11 @@ export default class Modal extends Component {
     portalClassName: PropTypes.string,
     bodyOpenClassName: PropTypes.string,
     className: PropTypes.oneOfType([
-      PropTypes.String,
+      PropTypes.string,
       PropTypes.object
     ]),
     overlayClassName: PropTypes.oneOfType([
-      PropTypes.String,
+      PropTypes.string,
       PropTypes.object
     ]),
     appElement: PropTypes.instanceOf(SafeHTMLElement),
diff --git a/src/components/ModalPortal.js b/src/components/ModalPortal.js
index 3c22c0ea..fbf27a81 100644
--- a/src/components/ModalPortal.js
+++ b/src/components/ModalPortal.js
@@ -31,11 +31,11 @@ export default class ModalPortal extends Component {
       overlay: PropTypes.object
     }),
     className: PropTypes.oneOfType([
-      PropTypes.String,
+      PropTypes.string,
       PropTypes.object
     ]),
     overlayClassName: PropTypes.oneOfType([
-      PropTypes.String,
+      PropTypes.string,
       PropTypes.object
     ]),
     onAfterOpen: PropTypes.func,