diff --git a/analysis/study_definition.py b/analysis/study_definition.py
index 5198b5c..283b6e0 100644
--- a/analysis/study_definition.py
+++ b/analysis/study_definition.py
@@ -13,7 +13,7 @@
 
 def number_of_episodes(start, end, selected_codelist):
     return {
-        f"episode_{start}": patients.with_these_clinical_events(
+        f"episode_{start.replace('-', '_')}": patients.with_these_clinical_events(
             codelist=selected_codelist,
             between=[start, end],
             episode_defined_as="series of events each <= 0 days apart",