diff --git a/Shorewall-core/configure.pl b/Shorewall-core/configure.pl
index 04159b6d2..b7d047caf 100755
--- a/Shorewall-core/configure.pl
+++ b/Shorewall-core/configure.pl
@@ -57,7 +57,7 @@ my $rcfile;
 my $rcfilename;
 
 unless ( defined $vendor ) {
-    if ( -f /etc/os-release) {
+    if ( -f '/etc/os-release' ) {
 	my $id = `cat /etc/os-release | grep ^ID`;
 
 	chomp $id;