diff --git a/src/CSSTransition.js b/src/CSSTransition.js
index 61592105..8fb69e19 100644
--- a/src/CSSTransition.js
+++ b/src/CSSTransition.js
@@ -190,7 +190,7 @@ class CSSTransition extends React.Component {
     const { doneClassName } = this.getClassNames('enter');
 
     if (type === 'appear' && phase === 'done' && doneClassName) {
-      className += ` ${doneClassName}`;
+      className = className ? `${className} ${doneClassName}` : doneClassName;
     }
 
     // This is to force a repaint,
diff --git a/test/CSSTransition-test.js b/test/CSSTransition-test.js
index 9ec4672a..b5addcd9 100644
--- a/test/CSSTransition-test.js
+++ b/test/CSSTransition-test.js
@@ -203,34 +203,37 @@ describe('CSSTransition', () => {
       expect(nodeRef.current.className).toBe('appear-test-enter-done');
     });
 
-    it('should not add undefined when appearDone is not defined', async () => {
-      const nodeRef = React.createRef();
-      let done = false;
-      render(
-        <CSSTransition
-          timeout={10}
-          nodeRef={nodeRef}
-          classNames={{ appear: 'appear-test' }}
-          in={true}
-          appear={true}
-          onEnter={(isAppearing) => {
-            expect(isAppearing).toEqual(true);
-            expect(nodeRef.current.className).toEqual('appear-test');
-          }}
-          onEntered={(isAppearing) => {
-            expect(isAppearing).toEqual(true);
-            expect(nodeRef.current.className).toEqual('');
-            done = true;
-          }}
-        >
-          <div ref={nodeRef} />
-        </CSSTransition>
-      );
-
-      await waitFor(() => {
-        expect(done).toEqual(true);
-      });
-    });
+    it.each(['', 'enter-done'])(
+      'should not add undefined when appearDone is omitted and enterDone is %s',
+      async (enterDone) => {
+        const nodeRef = React.createRef();
+        let done = false;
+        render(
+          <CSSTransition
+            timeout={10}
+            nodeRef={nodeRef}
+            classNames={{ appear: 'appear-test', enterDone }}
+            in={true}
+            appear={true}
+            onEnter={(isAppearing) => {
+              expect(isAppearing).toEqual(true);
+              expect(nodeRef.current.className).toEqual('appear-test');
+            }}
+            onEntered={(isAppearing) => {
+              expect(isAppearing).toEqual(true);
+              expect(nodeRef.current.className).toEqual(enterDone);
+              done = true;
+            }}
+          >
+            <div ref={nodeRef} />
+          </CSSTransition>
+        );
+
+        await waitFor(() => {
+          expect(done).toEqual(true);
+        });
+      }
+    );
 
     it('should not be appearing in normal enter mode', async () => {
       let count = 0;