diff --git a/test/typecheck/function_params_test.gleam b/test/typecheck/function_params_test.gleam
index c761c3f..77e4baa 100644
--- a/test/typecheck/function_params_test.gleam
+++ b/test/typecheck/function_params_test.gleam
@@ -2,6 +2,7 @@ import glance
 import gleam/option
 import gleeunit/should
 import glimpse/error
+import glimpse/internal/typecheck/environment
 import typecheck/helpers
 
 pub fn int_param_test() {
@@ -36,7 +37,18 @@ pub fn string_param_test() {
   |> should.equal(option.Some(glance.NamedType("String", option.None, [])))
 }
 
-pub fn incorrect_param_return_fails() {
+pub fn incorrect_param_return_fails_test() {
   helpers.error_function_typecheck("fn foo(a: String) -> Nil { a }")
   |> should.equal(error.InvalidReturnType("foo", "String", "Nil"))
 }
+
+pub fn custom_type_param_test() {
+  let function_out =
+    helpers.ok_function_env_typecheck(
+      environment.new() |> environment.add_custom_type("MyType"),
+      "fn foo(my_type: MyType) -> MyType { my_type }",
+    )
+
+  function_out.return
+  |> should.equal(option.Some(glance.NamedType("MyType", option.None, [])))
+}
diff --git a/test/typecheck/helpers.gleam b/test/typecheck/helpers.gleam
index 5787452..f97f823 100644
--- a/test/typecheck/helpers.gleam
+++ b/test/typecheck/helpers.gleam
@@ -38,12 +38,19 @@ pub fn glance_function(definition: String) -> glance.Function {
   definition.definition
 }
 
-pub fn ok_function_typecheck(definition: String) -> glance.Function {
+pub fn ok_function_env_typecheck(
+  env: Environment,
+  definition: String,
+) -> glance.Function {
   let function = glance_function(definition)
-  typecheck.function(environment.new(), function)
+  typecheck.function(env, function)
   |> should.be_ok
 }
 
+pub fn ok_function_typecheck(definition: String) -> glance.Function {
+  ok_function_env_typecheck(environment.new(), definition)
+}
+
 pub fn error_function_typecheck(definition: String) -> error.TypeCheckError {
   let function = glance_function(definition)
   typecheck.function(environment.new(), function)