diff --git a/fhem/fhem.pl b/fhem/fhem.pl
index a1934773e..b6b4c5b17 100755
--- a/fhem/fhem.pl
+++ b/fhem/fhem.pl
@@ -4226,6 +4226,9 @@ readingsEndUpdate($$)
         $value = "Error evaluating $name userReading $reading: $@";
         Log 1, $value;
         $result= $value;
+      } elsif(!defined($value)) {
+        warn("$name userReadings $reading evaluated to undef");
+        next;
       } elsif($modifier eq "none") {
         $result= $value;
       } elsif($modifier eq "difference") {