diff --git a/tests/extmod/select_poll_fd.py b/tests/extmod/select_poll_fd.py
index 3677ab5712..3f7b5aa738 100644
--- a/tests/extmod/select_poll_fd.py
+++ b/tests/extmod/select_poll_fd.py
@@ -35,7 +35,8 @@ poller.register(1, select.POLLIN)
 print(poller.poll(0))
 
 # Test registering a very large number of file descriptors (will trigger
-# EINVAL due to more than OPEN_MAX fds).
+# EINVAL due to more than OPEN_MAX fds). Typically it's 1024 (and on GitHub CI
+# we force this via `ulimit -n 1024`).
 poller = select.poll()
 for fd in range(6000):
     poller.register(fd)
diff --git a/tools/ci.sh b/tools/ci.sh
index d3ae411582..95b1f19ffc 100755
--- a/tools/ci.sh
+++ b/tools/ci.sh
@@ -6,6 +6,9 @@ else
     MAKEOPTS="-j$(sysctl -n hw.ncpu)"
 fi
 
+# Ensure known OPEN_MAX (NO_FILES) limit.
+ulimit -n 1024
+
 ########################################################################################
 # general helper functions