diff --git a/src/uproot/writing/_cascade.py b/src/uproot/writing/_cascade.py
index d790788b7..fc37e4a45 100644
--- a/src/uproot/writing/_cascade.py
+++ b/src/uproot/writing/_cascade.py
@@ -2159,8 +2159,10 @@ def deserialize(cls, raw_bytes, location):
                 uuid_bytes,
             ) = uproot.reading._file_header_fields_big.unpack(raw_bytes)
             assert units == 8
+            outversion = version - 1000000
         else:
             assert units == 4
+            outversion = version
 
         assert begin >= uproot.reading._file_header_fields_small.size
         assert free_location >= 0
@@ -2183,7 +2185,7 @@ def deserialize(cls, raw_bytes, location):
             info_num_bytes,
             uuid.UUID(bytes=uuid_bytes),
         )
-        out._version = version - 1000000
+        out._version = outversion
         out._begin = begin
         return out
 
diff --git a/tests/test_1212_dont_let_update_mess_up_file_version.py b/tests/test_1212_dont_let_update_mess_up_file_version.py
new file mode 100644
index 000000000..e5913a43a
--- /dev/null
+++ b/tests/test_1212_dont_let_update_mess_up_file_version.py
@@ -0,0 +1,18 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
+
+import os
+
+import uproot
+
+
+def test(tmp_path):
+    filename = os.path.join(tmp_path, "whatever.root")
+
+    with uproot.recreate(filename) as file:
+        file["one"] = "one"
+
+    with uproot.update(filename) as file:
+        file["two"] = "two"
+
+    with uproot.open(filename) as file:
+        assert file.file.fVersion == uproot.writing._cascade.FileHeader.class_version