diff --git a/gnssanalysis/gn_io/clk.py b/gnssanalysis/gn_io/clk.py
index e68fb81..1d164ff 100644
--- a/gnssanalysis/gn_io/clk.py
+++ b/gnssanalysis/gn_io/clk.py
@@ -1,4 +1,5 @@
 """RINEX CLK file parsing function"""
+
 import logging as _logging
 import re as _re
 from io import BytesIO as _BytesIO
@@ -21,7 +22,7 @@ def read_clk(clk_path):
 
     data = content[data_b:]
     data_line = _RE_LINE.search(data)
-    assert data_line is not None
+    assert data_line is not None, "CLK data not parsable - file could be empty"
 
     len_line = len(data_line.groups()[0])  # need to get a line and check the length