diff --git a/include/allegro_flare/attributes.h b/include/allegro_flare/attributes.h
index 9bdc220ff..67f291b90 100644
--- a/include/allegro_flare/attributes.h
+++ b/include/allegro_flare/attributes.h
@@ -144,6 +144,8 @@ class Attributes
    static std::vector<std::string> get_known_datatypes();
 
 private:
+   static std::vector<std::string> denied_custom_types;
+
    int __find_attribute_index(std::string key);
    int __find_attribute_index(std::string key, std::string value);
    int __find_or_create_attribute_index(std::string key);
diff --git a/src/attributes.cpp b/src/attributes.cpp
index 6154f26fd..49e8d655c 100644
--- a/src/attributes.cpp
+++ b/src/attributes.cpp
@@ -97,6 +97,9 @@ std::vector<DatatypeDefinition> DatatypeDefinition::definitions;
 
 
 
+std::vector<std::string> Attributes::denied_custom_types = {"int", "float", "bool", "string"};
+
+
 
 Attributes::Attributes()
 {
@@ -496,15 +499,17 @@ bool Attributes::pull_value(std::string key)
 
 
 bool Attributes::create_datatype_definition(
-   std::string datatype_identifier,
+   std::string identifier,
    bool (*to_val_func)(void *val, std::string str),
    std::string (*to_str_func)(void *val))
 {
-   DatatypeDefinition *definition = DatatypeDefinition::find_definition(datatype_identifier);
-   DatatypeDefinition::definitions.push_back(DatatypeDefinition(datatype_identifier, to_val_func, to_str_func));
+   std::vector<std::string> &denied_types = Attributes::denied_custom_types;
+   if (std::find(denied_types.begin(), denied_types.end(), identifier) != denied_types.end()) return false;
 
+   DatatypeDefinition *definition = DatatypeDefinition::find_definition(identifier);
    if (definition) return false;
-   DatatypeDefinition::definitions.push_back(DatatypeDefinition(datatype_identifier, to_val_func, to_str_func));
+
+   DatatypeDefinition::definitions.push_back(DatatypeDefinition(identifier, to_val_func, to_str_func));
    return true;
 }
 
@@ -521,8 +526,10 @@ bool Attributes::datatype_is_known(std::string datatype)
 
 std::vector<std::string> Attributes::get_known_datatypes()
 {
-   // TODO
-   std::vector<std::string> result;
+   std::vector<std::string> result = Attributes::denied_custom_types;
+   for (unsigned i=0; i<DatatypeDefinition::definitions.size(); i++)
+      result.push_back(DatatypeDefinition::definitions[i].identifier);
+   std::sort(result.begin(), result.end());
    return result;
 }
 
diff --git a/tests/attributes_test.cpp b/tests/attributes_test.cpp
index 7ba0d1a28..c27c6e32c 100644
--- a/tests/attributes_test.cpp
+++ b/tests/attributes_test.cpp
@@ -492,19 +492,20 @@ BOOST_AUTO_TEST_CASE(a_bound_attribute_can_be_unbound)
    BOOST_CHECK_EQUAL(attributes.is_bound("val"), false);
 }
 
-BOOST_AUTO_TEST_CASE(a_bound_attribute_will_pull_when_getting_from_a_custom_datatype)
+BOOST_AUTO_TEST_CASE(user_can_get_the_known_datatypes_in_alphabetical_order)
 {
-   // TODO
-}
+   Attributes attributes;
+   auto types = attributes.get_known_datatypes();
 
-BOOST_AUTO_TEST_CASE(user_can_get_the_bound_datatype_on_custom_datatypes)
-{
-   // TODO
-}
+   attributes.create_datatype_definition("my_custom_datatype",
+      my_custom_datatype::to_val_func, my_custom_datatype::to_str_func);
 
-BOOST_AUTO_TEST_CASE(user_can_get_the_known_datatypes)
-{
-   // TODO
+   BOOST_REQUIRE_EQUAL(types.size(), 5);
+   BOOST_CHECK_EQUAL(types[0], "bool");
+   BOOST_CHECK_EQUAL(types[1], "float");
+   BOOST_CHECK_EQUAL(types[2], "int");
+   BOOST_CHECK_EQUAL(types[3], "my_custom_datatype");
+   BOOST_CHECK_EQUAL(types[4], "string");
 }