diff --git a/lib/sympto/mucus.js b/lib/sympto/mucus.js
index f9c0d82..0931ad6 100644
--- a/lib/sympto/mucus.js
+++ b/lib/sympto/mucus.js
@@ -4,7 +4,7 @@ export default function (cycleDays) {
   const mucusPeak = mucusDays.find((day, i) => {
     if (day.mucus.value !== bestQuality) return false
 
-    const threeFollowingDays = cycleDays.slice(i + 1, i + 4)
+    const threeFollowingDays = mucusDays.slice(i + 1, i + 4)
     if (threeFollowingDays.length < 3) return false
 
     return threeFollowingDays.every(day => day.mucus.value < bestQuality)