diff --git a/src/commonMain/kotlin/de/kif/common/ConstraintChecking.kt b/src/commonMain/kotlin/de/kif/common/ConstraintChecking.kt
index 7e9c7a8..78e2ee8 100644
--- a/src/commonMain/kotlin/de/kif/common/ConstraintChecking.kt
+++ b/src/commonMain/kotlin/de/kif/common/ConstraintChecking.kt
@@ -67,8 +67,8 @@ fun checkConstraints(
                 }
 
                 ConstraintType.NotOnDay -> {
-                    val notOnDay = constraints.map { it.day == schedule.day }
-                    if (notOnDay.any()) {
+                    val notOnDay = constraints.map { it.day != schedule.day }
+                    if (notOnDay.none()) {
                         val dayList = constraints.mapNotNull { it.day }.distinct().sorted()
                         errors += ConstraintError("Work group requires not days $dayList, but is on ${schedule.day}!")
                     }