diff --git a/fhem/fhem.pl b/fhem/fhem.pl
index 575c7c371..0772743ee 100755
--- a/fhem/fhem.pl
+++ b/fhem/fhem.pl
@@ -3974,6 +3974,7 @@ ReadingsNum($$$;$)
 {
   my ($d,$n,$default,$round) = @_;
   my $val = ReadingsVal($d,$n,$default);
+  return undef if(!defined($val));
   $val = ($val =~ /(-?\d+(\.\d+)?)/ ? $1 : "");
   $val = round($val,$round) if($round);
   return $val;