diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index e01a4ba..df1d8ac 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -199,7 +199,11 @@ sub lisp_format {
 
 sub lisp_equal {
   my ($a, $b) = @_;
-  if ( ref($a) eq "ARRAY" && ref($b) eq "ARRAY" )
+  if ( !defined($a) && !defined($b) )
+    {
+      return LISP_TRUE;
+    }
+  elsif ( ref($a) eq "ARRAY" && ref($b) eq "ARRAY" )
     {
       return LISP_FALSE unless scalar(@$a) == scalar(@$b);