diff --git a/fhem/CHANGED b/fhem/CHANGED
index a370b2b07..e1142e1ab 100644
--- a/fhem/CHANGED
+++ b/fhem/CHANGED
@@ -1,5 +1,6 @@
 # Add changes at the top of the list. Keep it in ASCII, and 80-char wide.
 # Do not insert empty lines here, update check depends on it.
+ - bugfix:  20_FRM_IN: fix undefined value warning when not initialized
  - changed: 74_Unifi: removed UCv3 support
  - feature: 59_Weather: and APIs fix utf8 encode bug, insert patch from lippie
  - change:  98_Text2Speech.pm: removed some bugs, thanks to RomanticBoy83
diff --git a/fhem/FHEM/20_FRM_IN.pm b/fhem/FHEM/20_FRM_IN.pm
index cf759590f..33cf0b976 100755
--- a/fhem/FHEM/20_FRM_IN.pm
+++ b/fhem/FHEM/20_FRM_IN.pm
@@ -127,7 +127,10 @@ FRM_IN_observer($$$$)
 {
 	my ($pin,$last,$new,$hash) = @_;
 	my $name = $hash->{NAME};
-	my $old = ReadingsVal($name, "reading", undef) eq "on" ? PIN_HIGH : PIN_LOW;
+	my $old = ReadingsVal($name, "reading", undef);
+	if (defined($old)) {
+		$old = $old eq "on" ? PIN_HIGH : PIN_LOW;
+	}
 	if (AttrVal($hash->{NAME},"activeLow","no") eq "yes") {
 		$new = $new == PIN_LOW ? PIN_HIGH : PIN_LOW;
 	}
@@ -308,6 +311,9 @@ FRM_IN_Attr($$$$) {
 
   CHANGES
 
+  15.02.2019 jensb
+    o bugfix: change detection no longer assumes that reading "reading" is defined
+    
   04.11.2018 jensb
     o bugfix: get alarm/reading/state
     o feature: remove unused FHEMWEB input field from all get commands