diff --git a/src/typechecker.ts b/src/typechecker.ts
index 47d1e1e..b1c8994 100644
--- a/src/typechecker.ts
+++ b/src/typechecker.ts
@@ -361,7 +361,7 @@ export function typecheck(defs: Definition[], typeDefs: TypeDefinition[] = [], c
             } else {
                 let fnType: TypeAST = resultType;
                 for (let i = ctor.args.length - 1; i >= 0; i--) {
-                    fnType = { kind: 'type-function', param: ctor.args[0], result: fnType };
+                    fnType = { kind: 'type-function', param: ctor.args[i], result: fnType };
                 }
                 env.set(ctor.name, fnType);
             }