diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index 41be544..bcce925 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -80,8 +80,17 @@ sub tokenize {
       elsif($str =~ s/^([^\s()"]+)//)
         {
           my $ident = $1;
-          if($ident =~ /^[0-9]+$/)
+          if($ident eq ".")
             {
+              die "short cons not supported"
+            }
+          elsif($ident =~ /^-?([0-9]+|[0-9]*\.[0-9]*)$/)
+            {
+              if($ident =~ s/^-//)
+                {
+                  $ident = 0 - $ident;
+                }
+
               push @tokens, {
                 type => NUMBER,
                 value => 0+ $ident,