diff --git a/lib/gitem/api.py b/lib/gitem/api.py
index 007b5d4..1910291 100644
--- a/lib/gitem/api.py
+++ b/lib/gitem/api.py
@@ -169,9 +169,17 @@ def get_users_public_repositories(self, username, type=None, sort=None, directio
 
         https://developer.github.com/v3/repos/#list-user-repositories
         """
-        assert type in ["all", "owner", "member", None]
-        assert sort in ["created", "updated", "pushed", "full_name", None]
-        assert direction in ["asc", "desc", None]
+        type_values = ["all", "owner", "member"]
+        if type not in type_values and type is not None:
+            raise ValueError("type must be one of {}".format(type_values))
+
+        sort_values = ["created", "updated", "pushed", "full_name"]
+        if sort not in sort_values and sort is not None:
+            raise ValueError("sort must be one of {}".format(sort_values))
+
+        direction_values = ["asc", "desc"]
+        if direction not in direction_values and direction is not None:
+            raise ValueError("direction must be one of {}".format(direction_values))
 
         method = "GET"
         endpoint = "/users/{}/repos".format(username)
@@ -218,7 +226,9 @@ def get_organizations_public_repositories(self, organization, type=None):
 
         https://developer.github.com/v3/repos/#list-organization-repositories
         """
-        assert type in ["all", "public", "private", "forks", "sources", "member", None]
+        type_values = ["all", "public", "private", "forks", "sources", "member"]
+        if type not in type_values and type is not None:
+            raise ValueError("type must be one of {}".format(type_values))
 
         method = "GET"
         endpoint = "/orgs/{}/repos".format(organization)
@@ -265,7 +275,9 @@ def get_repository_contributors(self, owner, repository, anon=None):
 
         https://developer.github.com/v3/repos/#list-contributors
         """
-        assert anon in [1, "true", None]
+        anon_values = [1, "true"]
+        if anon not in anon_values and type is not None:
+            raise ValueError("anon must be one of {}".format(anon_values))
 
         method = "GET"
         endpoint = "/repos/{}/{}/contributors".format(owner, repository)
diff --git a/tests/test_api.py b/tests/test_api.py
index a5cfced..d167a66 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -202,6 +202,41 @@ def test_paged_ok(self):
             self.assertOk(status_code)
             self.assertEqual(result, expected)
 
+    def test_get_users_public_repositories_bad_type(self):
+        type = ""
+        ghapi = api.Api()
+
+        with self.assertRaises(ValueError):
+            ghapi.get_users_public_repositories("UNUSED", type=type)
+
+    def test_get_users_public_repositories_bad_sort(self):
+        sort = ""
+        ghapi = api.Api()
+
+        with self.assertRaises(ValueError):
+            ghapi.get_users_public_repositories("UNUSED", sort=sort)
+
+    def test_get_users_public_repositories_bad_direction(self):
+        direction = ""
+        ghapi = api.Api()
+
+        with self.assertRaises(ValueError):
+            ghapi.get_users_public_repositories("UNUSED", direction=direction)
+
+    def test_get_organizations_public_repositories_bad_type(self):
+        type = ""
+        ghapi = api.Api()
+
+        with self.assertRaises(ValueError):
+            ghapi.get_organizations_public_repositories("UNUSED", type=type)
+
+    def test_get_repository_contributors_bad_anon(self):
+        anon = ""
+        ghapi = api.Api()
+
+        with self.assertRaises(ValueError):
+            ghapi.get_repository_contributors("UNUSED", "UNUSED", anon=anon)
+
 
 if __name__ == "__main__":
     unittest.main()