diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index 4d87069..6e360bf 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -157,7 +157,7 @@ sub ctx_assign {
       $ctx = $ctx->{base};
     }
 
-  die "Variable $identifier does not defined" unless defined $ctx;
+  die "Variable $identifier is not defined" unless defined $ctx;
 
   $ctx->{vars}->{$identifier} = $value;
 }