diff --git a/lib/sympto-adapter.js b/lib/sympto-adapter.js
index f2fb1c0..5caf8ea 100644
--- a/lib/sympto-adapter.js
+++ b/lib/sympto-adapter.js
@@ -113,6 +113,10 @@ function formatCycleForSympto(cycle) {
     if (hasIncompleteCervixValue(day)) {
       delete day.cervix
     }
+    // remove days with incomplete mucus value (because nfp-mucus returns null when that's the case)
+    if (day.mucus.value === null) {
+      delete day.mucus
+    }
     // change format
     ['bleeding', 'temperature', 'mucus'].forEach(symptomName => {
       if (day[symptomName]) day[symptomName] = day[symptomName].value