1 files changed, 3 insertions, 0 deletions
diff --git a/src/type/mod.rs b/src/type/mod.rs
index eba817b..9c2f89c 100644
--- a/src/type/mod.rs
+++ b/src/type/mod.rs
@@ -23,6 +23,8 @@ pub enum Type {
 
     List(Vec<Type>),
 
+    NilType,
+
     VecOf(Box<Type>),
     VecType, // constructor
 
@@ -108,6 +110,7 @@ impl Type {
             QuoteTy => Ok(()),
             VecType => Ok(()),
             LetType => Ok(()),
+	    NilType => Ok(()),
             Arrow(a, b) => b.is_concrete().and_then(|_ok| a.is_concrete()),
             List(v) => {
                 let mut res = Ok(());