diff --git a/tests/test_signatures.py b/tests/test_signatures.py
index 5e4fdde..da9fb99 100644
--- a/tests/test_signatures.py
+++ b/tests/test_signatures.py
@@ -228,8 +228,19 @@ def test_store_signatures_with_different_digests(
     )
 
     # Mock get_last_log_index
+    mocker.patch(
+        "dangerzone.updater.signatures.get_last_log_index",
+        return_value=50,
+    )
+
+    # Call store_signatures
+    with pytest.raises(errors.SignatureMismatch):
+        store_signatures(signatures, image_digest, TEST_PUBKEY_PATH)
+
     # Verify that the signatures file was not created
     assert not (signatures_path / f"{image_digest}.json").exists()
+
+    # Verify that the log index file was not updated
     assert not (signatures_path / "last_log_index").exists()