diff --git a/git-difftool.perl b/git-difftool.perl
index 0a90de414..12231fbc6 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -336,7 +336,7 @@ sub main
 	}
 	if ($opts{gui}) {
 		my $guitool = Git::config('diff.guitool');
-		if (length($guitool) > 0) {
+		if (defined($guitool) && length($guitool) > 0) {
 			$ENV{GIT_DIFF_TOOL} = $guitool;
 		}
 	}