diff --git a/bin/compose-core b/bin/compose-core
index 7ae5405..d32dac3 100755
--- a/bin/compose-core
+++ b/bin/compose-core
@@ -2611,7 +2611,8 @@ get_all_relations () {
                     return 1
                     ;;
             esac
-            constraint=$(echo "$relation_def" | shyaml get-value constraint optional 2>/dev/null)
+            constraint=$(echo "$relation_def" | shyaml get-value constraint 2>/dev/null)
+            constraint=${constraint:-optional}
             case "$constraint" in
                 "required")
                     required+=("$service" "$relation_name" "$relation_def")