diff --git a/src/lists.coffee b/src/lists.coffee
index 94fd3ce..0e4a3cf 100644
--- a/src/lists.coffee
+++ b/src/lists.coffee
@@ -35,7 +35,7 @@ _annotate = (ConsList) ->
   
   cons = (a = nil, b = nil) ->
     return (new ConsList()) if (nilp a) and (nilp b)
-    if (a) then (new ConsList(a, b)) else (new ConsList(b))
+    if (a?) then (new ConsList(a, b)) else (new ConsList(b))
   
   nil = (-> new ConsList())()
   
diff --git a/test/lists.coffee b/test/lists.coffee
index aefa43b..a7f026a 100644
--- a/test/lists.coffee
+++ b/test/lists.coffee
@@ -9,6 +9,7 @@ expect = chai.expect
 describe "Basic list building", ->
   for [t, v] in [
     [cons(), cons()]
+    [cons(0), [0, nil]]
     [cons(nil), cons(nil)]
     [cons('a'), cons('a')]
     [cons('a', cons('b', cons('c'))), cons('a', cons('b', cons('c')))]