diff --git a/stdlib/@tests/stubtest_allowlists/linux-py38.txt b/stdlib/@tests/stubtest_allowlists/linux-py38.txt
index ee1730e17..650fab4ea 100644
--- a/stdlib/@tests/stubtest_allowlists/linux-py38.txt
+++ b/stdlib/@tests/stubtest_allowlists/linux-py38.txt
@@ -1,3 +1,5 @@
+# `eventmask` argument exists at runtime, but is not correctly recognized
+# while being inspected by stubtest. Fixed in Python 3.10.
 select.epoll.register
 
 # Exists at runtime, but missing from stubs
diff --git a/stdlib/@tests/stubtest_allowlists/linux-py39.txt b/stdlib/@tests/stubtest_allowlists/linux-py39.txt
index ee1730e17..650fab4ea 100644
--- a/stdlib/@tests/stubtest_allowlists/linux-py39.txt
+++ b/stdlib/@tests/stubtest_allowlists/linux-py39.txt
@@ -1,3 +1,5 @@
+# `eventmask` argument exists at runtime, but is not correctly recognized
+# while being inspected by stubtest. Fixed in Python 3.10.
 select.epoll.register
 
 # Exists at runtime, but missing from stubs