diff --git a/src/data.hpp b/src/data.hpp
index 256a7e4..dc049b5 100644
--- a/src/data.hpp
+++ b/src/data.hpp
@@ -7,6 +7,6 @@ namespace lily {
     struct constructor {
         int parent_type;
         int tag;
-        std::vector<type_ptr> params;
+        std::vector<std::weak_ptr<type>> params;
     };
 }
diff --git a/src/main.cpp b/src/main.cpp
index 8298a1e..7afe916 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -7,7 +7,7 @@ int main() {
         lily::parse(
                 "data Bool = { True, False }\n"
                 "data Color = { Red, Black }\n"
-                "data IntList = { Nil, Cons(Int, Int) }\n"
+                "data IntList = { Nil, Cons(Int, IntList) }\n"
                 "defn add x y = { x + y }");
     } catch(lily::error& e) {
         std::cout << e.message << std::endl;
diff --git a/src/parser.cpp b/src/parser.cpp
index 4892307..7223edd 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -167,10 +167,10 @@ namespace lily {
         return prog.types[str];
     }
 
-    static void collect_type_params(program& prog, std::vector<type_ptr>& into, pgs_tree* params, const char* source) {
+    static void collect_type_params(program& prog, std::vector<std::weak_ptr<type>>& into, pgs_tree* params, const char* source) {
         while(true) {
             pgs_tree* param = PGS_TREE_NT_CHILD(*params, 0);
-            into.push_back(type_tree(prog, param, source));
+            into.push_back(std::weak_ptr<type>(type_tree(prog, param, source)));
 
             if(PGS_TREE_NT_COUNT(*params) == 1) break;
             params = PGS_TREE_NT_CHILD(*params, 2);
@@ -213,9 +213,9 @@ namespace lily {
             if(prog.types.count(data_name)) throw error("cannot redefine type");
 
             int new_type = prog.next_free_type_id++;
+            prog.types[data_name] = type_ptr(new type_int(new_type));
             collect_constructors(prog, new_type,
                     PGS_TREE_NT_CHILD(*PGS_TREE_NT_CHILD(*def, PGS_TREE_NT_COUNT(*def) - 1), 1), source);
-            prog.types[data_name] = type_ptr(new type_int(new_type));
         }
     }