diff --git a/bin/compose-core b/bin/compose-core
index f52a7de..ea6e425 100755
--- a/bin/compose-core
+++ b/bin/compose-core
@@ -1582,7 +1582,7 @@ relation-set () {
         return 1
     fi
 
-    _config_merge "$RELATION_DATA_FILE" <(echo "$key: $value")
+    _config_merge "$RELATION_DATA_FILE" <(yaml_key_val_str "$key" "$value")
 }
 export -f relation-set