diff --git a/src/borg/compress.pyx b/src/borg/compress.pyx
index e259c3748..d1703f16d 100644
--- a/src/borg/compress.pyx
+++ b/src/borg/compress.pyx
@@ -438,8 +438,16 @@ class ZLIB_legacy(CompressorBase):
 
     def decompress(self, meta, data):
         # note: for compatibility no super call, do not strip ID bytes
+        assert self.legacy_mode  # only borg 1.x repos have the legacy ZLIB format
+        assert meta is None
+        meta = {}
+        meta["ctype"] = ZLIB.ID  # change to non-legacy ZLIB id
+        meta["clevel"] = 255  # we do not know the compression level
+        meta["csize"] = len(data)
         try:
-            return meta, zlib.decompress(data)
+            data = zlib.decompress(data)
+            self.check_fix_size(meta, data)
+            return meta, data
         except zlib.error as e:
             raise DecompressionError(str(e)) from None
 
diff --git a/src/borg/testsuite/compress.py b/src/borg/testsuite/compress.py
index ddcb2267a..c294b5ff8 100644
--- a/src/borg/testsuite/compress.py
+++ b/src/borg/testsuite/compress.py
@@ -50,7 +50,7 @@ def test_lz4_buffer_allocation(monkeypatch):
 @pytest.mark.parametrize("invalid_cdata", [b"\xff\xfftotalcrap", b"\x08\x00notreallyzlib"])
 def test_autodetect_invalid(invalid_cdata):
     with pytest.raises(ValueError):
-        Compressor(**params, legacy_mode=True).decompress({}, invalid_cdata)
+        Compressor(**params, legacy_mode=True).decompress(None, invalid_cdata)
 
 
 def test_zlib_legacy_compat():
@@ -61,7 +61,7 @@ def test_zlib_legacy_compat():
         meta1, cdata1 = c.compress({}, DATA)
         cdata2 = zlib.compress(DATA, level)
         assert cdata1 == cdata2
-        meta2, data2 = c.decompress({}, cdata2)
+        meta2, data2 = c.decompress(None, cdata2)
         assert DATA == data2