diff --git a/saqc/__main__.py b/saqc/__main__.py
index 0b1a5c84b101abeed7efadabf63ad79146fd9887..34cfb73cebdb146152c243bd76c0d1c62ed153a4 100644
--- a/saqc/__main__.py
+++ b/saqc/__main__.py
@@ -51,7 +51,7 @@ def readData(reader_dict, fname):
     reader = reader_dict.get(extension)
     if not reader:
         raise ValueError(
-            f"Unsupported file format '{extension}', use one of {tuple(reader.keys())}"
+            f"Unsupported file format '{extension}', use one of {tuple(reader_dict.keys())}"
         )
     return reader(fname)
 
@@ -61,7 +61,7 @@ def writeData(writer_dict, df, fname):
     writer = writer_dict.get(extension)
     if not writer:
         raise ValueError(
-            f"Unsupported file format '{extension}', use one of {tuple(writer.keys())}"
+            f"Unsupported file format '{extension}', use one of {tuple(writer_dict.keys())}"
         )
     writer(df, fname)
 
diff --git a/tests/test__main__.py b/tests/test__main__.py
new file mode 100644
index 0000000000000000000000000000000000000000..a1fce8f1b89afe1d96c90bcf0f3d759f11c9d043
--- /dev/null
+++ b/tests/test__main__.py
@@ -0,0 +1,14 @@
+#!/usr/bin/env python
+import pytest
+import numpy as np
+import pandas as pd
+
+
+def test_unknownFileExtention():
+    from saqc.__main__ import readData, writeData, setupIO
+
+    reader, writer = setupIO(np.nan)
+    with pytest.raises(ValueError):
+        readData(reader, "foo.unknown")
+    with pytest.raises(ValueError):
+        writeData(reader, pd.DataFrame(), "foo.unknown")