diff --git a/apistar/schemas/openapi.py b/apistar/schemas/openapi.py
index b8742699..c2c31640 100644
--- a/apistar/schemas/openapi.py
+++ b/apistar/schemas/openapi.py
@@ -1,7 +1,6 @@
 import re
-from urllib.parse import urljoin
-
 import typesystem
+
 from apistar.document import Document, Field, Link, Section
 from apistar.schemas.jsonschema import JSON_SCHEMA
 
@@ -462,7 +461,7 @@ def get_link(
 
         return Link(
             name=name,
-            url=urljoin(base_url, path),
+            url=(base_url or "") + path,
             method=operation,
             title=title,
             description=description,
diff --git a/requirements.txt b/requirements.txt
index cf3a4712..88e4409f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,7 +3,7 @@ click
 jinja2
 requests
 pyyaml
-typesystem>=0.2.0
+typesystem==0.2.4
 
 # Testing requirements
 black
diff --git a/tests/schemas/test_openapi.py b/tests/schemas/test_openapi.py
index 3b23cafe..60fd9f5c 100644
--- a/tests/schemas/test_openapi.py
+++ b/tests/schemas/test_openapi.py
@@ -1,5 +1,4 @@
 import os
-
 import pytest
 
 import apistar
@@ -21,4 +20,7 @@ def test_openapi(filename):
 
     path, extension = os.path.splitext(filename)
     encoding = {".json": "json", ".yaml": "yaml"}[extension]
-    apistar.validate(content, format="openapi", encoding=encoding)
+    document = apistar.validate(content, format="openapi", encoding=encoding)
+    if document.url is not None:
+        for link_info in document.walk_links():
+            assert document.url in link_info.link.url