diff --git a/git_time_tracker/__main__.py b/git_time_tracker/__main__.py
index 8391c53..52a195b 100644
--- a/git_time_tracker/__main__.py
+++ b/git_time_tracker/__main__.py
@@ -20,7 +20,8 @@ def filter_samples(
     sample_filter: set[str],
 ) -> list[ActivitySample]:
     if not sample_filter:
-        raise ValueError('sample_filter must not be empty')
+        error_message = 'sample_filter must not be empty'
+        raise ValueError(error_message)
     return [s for s in samples if set(s.labels).intersection(sample_filter)]