diff --git a/core/assets/run.sh b/core/assets/run.sh
index 2d1ac5d..034ccbe 100755
--- a/core/assets/run.sh
+++ b/core/assets/run.sh
@@ -95,6 +95,9 @@ EOF
 cat > /etc/postfix-mailman.cfg <<EOF
 [postfix]
 transport_file_type: regex
+# While in regex mode, postmap_command is never used, a placeholder
+# is added here so that it doesn't break anything.
+postmap_command: true
 EOF