diff --git a/lib/ast.js b/lib/ast.js
index 5af060d7..9bdbe7bf 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -585,10 +585,11 @@ var AST_New = DEFNODE("New", null, {
 var AST_Seq = DEFNODE("Seq", "car cdr", {
     $documentation: "A sequence expression (two comma-separated expressions)",
     $propdoc: {
-        car: "[AST_Node] first element in sequence",
-        cdr: "[AST_Node] second element in sequence"
+        car: "[AST_Node] first element in sequence (should not be instanceof AST_Seq or null)",
+        cdr: "[AST_Node] second element in sequence (should not be null)"
     },
     $cons: function(x, y) {
+        // TODO: Remove these sanity checks at the end of PR
         if (!x) {
             throw new Error('AST_Seq.car missing');
         }