diff --git a/FU.pm b/FU.pm
index c21bb7f..37f90db 100644
--- a/FU.pm
+++ b/FU.pm
@@ -728,7 +728,7 @@ sub reset {
 
 sub _validate_header($hdr, $val) {
     confess "Invalid response header '$hdr'" if $hdr !~ /^$FU::hdrname_re$/;
-    confess "Invalid attempt to set response header containing a newline" if $val =~ /[\r\n]/;
+    confess "Invalid attempt to set response header containing a newline" if defined $val && $val =~ /[\r\n]/;
 }
 
 sub add_header($, $hdr, $val) {