diff --git a/lib/Zarn/Engine/Source_to_Sink.pm b/lib/Zarn/Engine/Source_to_Sink.pm
index c4d07c9..dc9c714 100644
--- a/lib/Zarn/Engine/Source_to_Sink.pm
+++ b/lib/Zarn/Engine/Source_to_Sink.pm
@@ -20,7 +20,7 @@ package Zarn::Engine::Source_to_Sink {
         );
 
         if ($ast && $rules) {
-            foreach my $token (@{$ast -> find('PPI::Token')}) {
+            foreach my $token (@{$ast -> find('PPI::Token') || []}) {
                 foreach my $rule (@{$rules}) {
                     my @sample   = $rule -> {sample} -> @*;
                     my $category = $rule -> {category};
@@ -63,4 +63,4 @@ package Zarn::Engine::Source_to_Sink {
     }
 }
 
-1;
\ No newline at end of file
+1;