diff --git a/aiida/schedulers/plugins/direct.py b/aiida/schedulers/plugins/direct.py
index 7417e621e9..c13f2059d8 100644
--- a/aiida/schedulers/plugins/direct.py
+++ b/aiida/schedulers/plugins/direct.py
@@ -55,6 +55,11 @@
     'W': JobState.RUNNING,
     'X': JobState.DONE,
     'Z': JobState.DONE,
+    '?': JobState.UNDETERMINED,
+    # `ps` can sometimes return `?` for the state of a process on macOS. This corresponds to an "unknown" state, see:
+    #
+    # https://apple.stackexchange.com/q/460394/497071
+    #
     # Not sure about these three, I comment them out (they used to be in
     # here, but they don't appear neither on ubuntu nor on Mac)
     #    'F': JobState.DONE,