diff --git a/lib/sympto/cervix.js b/lib/sympto/cervix.js
index 1f012b3251bfafd12405a9a42213fdf404a77a69..15c990a2260c4d1dcaaebd5b780aba266894b1ac 100644
--- a/lib/sympto/cervix.js
+++ b/lib/sympto/cervix.js
@@ -2,6 +2,7 @@ export default function (cycleDays, tempEvalEndIndex) {
   const notDetected = { detected: false }
   const cervixDays = cycleDays
     .filter(day => day.cervix && !day.cervix.exclude)
+    .filter(day => typeof day.cervix.opening === 'number' && typeof day.cervix.firmness === 'number')
 
   // we search for the day of cervix peak, which must:
   // * have fertile cervix values