diff --git a/src/forms/CalendarForm/validation/validateExceptions.tsx b/src/forms/CalendarForm/validation/validateExceptions.tsx
index 01c7ed17..425367a9 100644
--- a/src/forms/CalendarForm/validation/validateExceptions.tsx
+++ b/src/forms/CalendarForm/validation/validateExceptions.tsx
@@ -249,10 +249,10 @@ export function validateExceptionInterOverlaps(
     rows.map((row) => ({
       i: row.i,
       startDate: dayjs
-        .min(row.rows.map(({ startDate }) => dayjs(startDate)))
+        .min(row.rows.map(({ startDate }) => dayjs?.(startDate)))
         .startOf('day'),
       endDate: dayjs
-        .max(row.rows.map(({ endDate }) => dayjs(endDate)))
+        .max(row.rows.map(({ endDate }) => dayjs?.(endDate)))
         .endOf('day')
     }));