diff --git a/lib/date.js b/lib/date.js
index 63bad45e..da071128 100644
--- a/lib/date.js
+++ b/lib/date.js
@@ -136,6 +136,10 @@ export function getDateArray(data, startDate, endDate, unit) {
 }
 
 export function getDateLength(startDate, endDate, unit) {
+  if (!unit) {
+    unit = 'day';
+  }
+
   const [diff] = dateFuncs[unit];
   return diff(endDate, startDate) + 1;
 }