diff --git a/src/borg/repoobj.py b/src/borg/repoobj.py
index 14a575de5..8c47b1c7c 100644
--- a/src/borg/repoobj.py
+++ b/src/borg/repoobj.py
@@ -35,7 +35,6 @@ class RepoObj:
     ) -> bytes:
         assert isinstance(id, bytes)
         assert isinstance(meta, dict)
-        meta = dict(meta)  # make a copy, so call arg is not modified
         assert isinstance(data, (bytes, memoryview))
         assert compress or size is not None and ctype is not None and clevel is not None
         if compress:
@@ -115,14 +114,22 @@ class RepoObj1:  # legacy
     def id_hash(self, data: bytes) -> bytes:
         return self.key.id_hash(data)
 
-    def format(self, id: bytes, meta: dict, data: bytes, compress: bool = True, size: int = None) -> bytes:
+    def format(
+        self,
+        id: bytes,
+        meta: dict,
+        data: bytes,
+        compress: bool = True,
+        size: int = None,
+        ctype: int = None,
+        clevel: int = None,
+    ) -> bytes:
         assert isinstance(id, bytes)
         assert meta == {}
         assert isinstance(data, (bytes, memoryview))
-        assert compress or size is not None
-        assert compress or size is not None
+        assert compress or size is not None and ctype is not None and clevel is not None
         if compress:
-            assert size is None
+            assert size is None or size == len(data)
             meta, data_compressed = self.compressor.compress(meta, data)
         else:
             assert isinstance(size, int)
@@ -130,6 +137,9 @@ class RepoObj1:  # legacy
         data_encrypted = self.key.encrypt(id, data_compressed)
         return data_encrypted
 
+    def parse_meta(self, id: bytes, cdata: bytes) -> dict:
+        raise NotImplementedError("parse_meta is not available for RepoObj1")
+
     def parse(self, id: bytes, cdata: bytes, decompress: bool = True) -> tuple[dict, bytes]:
         assert isinstance(id, bytes)
         assert isinstance(cdata, bytes)
diff --git a/src/borg/testsuite/repoobj.py b/src/borg/testsuite/repoobj.py
index b67b56e7c..1f3d17f2e 100644
--- a/src/borg/testsuite/repoobj.py
+++ b/src/borg/testsuite/repoobj.py
@@ -78,7 +78,7 @@ def test_borg1_borg2_transition(key):
     # note: as we did not decompress, we do not have "size" and we need to get it from somewhere else.
     # here, we just use len_data. for borg transfer, we also know the size from another metadata source.
     borg2_cdata = repo_objs2.format(
-        id, meta1, compr_data1[2:], compress=False, size=len_data, ctype=meta1["ctype"], clevel=meta1["clevel"]
+        id, dict(meta1), compr_data1[2:], compress=False, size=len_data, ctype=meta1["ctype"], clevel=meta1["clevel"]
     )
     meta2, data2 = repo_objs2.parse(id, borg2_cdata)
     assert data2 == data