diff --git a/swh/loader/cvs/cvsclient.py b/swh/loader/cvs/cvsclient.py
--- a/swh/loader/cvs/cvsclient.py
+++ b/swh/loader/cvs/cvsclient.py
@@ -339,9 +339,12 @@
             if response is None:
                 raise CVSProtocolError("No response from CVS server")
             if response[0:2] == b"E ":
-                if len(path) > 0 and response[-11:] == b" - ignored\n":
+                if len(path) > 0 and (
+                    response.endswith(b" - ignored\n")
+                    or b"could not read RCS file" in response
+                ):
                     response = self.conn_read_line()
-                    if response != b"error  \n":
+                    if response not in (b"error  \n", b"ok\n"):
                         raise CVSProtocolError(
                             "Invalid response from CVS server: %s" % response
                         )
diff --git a/swh/loader/cvs/tests/test_cvsclient.py b/swh/loader/cvs/tests/test_cvsclient.py
new file mode 100644
--- /dev/null
+++ b/swh/loader/cvs/tests/test_cvsclient.py
@@ -0,0 +1,30 @@
+# Copyright (C) 2022  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU Affero General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from urllib.parse import urlparse
+
+from swh.loader.cvs.cvsclient import CVSClient
+
+
+def test_cvs_client_rlog_could_not_read_rcs_file(mocker):
+
+    url = "ssh://anoncvs@anoncvs.example.org/cvsroot/src"
+    file = "src/README.txt"
+
+    mocker.patch("swh.loader.cvs.cvsclient.socket")
+    conn_read_line = mocker.patch.object(CVSClient, "conn_read_line")
+    conn_read_line.side_effect = [
+        # server response lines when client is initialized
+        b"Valid-requests ",
+        b"ok\n",
+        # server response lines when attempting to fetch rlog of a file
+        # but RCS file is missing
+        f"E cvs rlog: could not read RCS file for {file}\n".encode(),
+        b"ok\n",
+    ]
+
+    client = CVSClient(urlparse(url))
+
+    assert client.fetch_rlog(file.encode()) is None