diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index e55b3ec1a..dd13ef4ce 100644
--- a/src/_pytest/pytester.py
+++ b/src/_pytest/pytester.py
@@ -1349,7 +1349,7 @@ class Pytester:
                 stderr=f2,
                 close_fds=(sys.platform != "win32"),
             )
-            if isinstance(stdin, bytes):
+            if popen.stdin is not None:
                 popen.stdin.close()
 
             def handle_timeout() -> None: