diff --git a/check-types/index.ts b/check-types/index.ts
index d8a3874..8f87885 100644
--- a/check-types/index.ts
+++ b/check-types/index.ts
@@ -1,4 +1,4 @@
-import { SimpleNode, SimpleDocument, SimpleElement, SimpleText, SimpleComment, SimpleDocumentFragment, SerializableNode, SerializableElement } from '@simple-dom/interface';
+import { SimpleDocument, SimpleElement, SimpleText, SimpleComment, SimpleDocumentFragment, SerializableNode, SerializableElement } from '@simple-dom/interface';
 
 export class CheckSimple {
   createHTMLDocument(): SimpleDocument {
@@ -20,6 +20,10 @@ export class CheckSimple {
   createDocumentFragment(text: string): SimpleDocumentFragment {
     return document.createDocumentFragment() as SimpleDocumentFragment;
   }
+
+  getOffsetParent(): SimpleElement | null {
+    return document.createElement('div').offsetParent as SimpleElement | null;
+  }
 }
 
 export class CheckSerializable {
diff --git a/packages/@simple-dom/interface/index.d.ts b/packages/@simple-dom/interface/index.d.ts
index b86e0d6..dcce1c3 100644
--- a/packages/@simple-dom/interface/index.d.ts
+++ b/packages/@simple-dom/interface/index.d.ts
@@ -47,7 +47,7 @@ export type SimpleNode =
   SimpleDocumentFragment;
 
 export interface SimpleNodeBase {
-  readonly ownerDocument: SimpleDocument;
+  readonly ownerDocument: SimpleDocument | null;
   readonly nodeType: NodeType;
   readonly nodeName: string;
 
diff --git a/testem.json b/testem.json
index c2c9aa1..a9f993c 100644
--- a/testem.json
+++ b/testem.json
@@ -1,7 +1,6 @@
 {
   "test_page": "test/index.html",
   "parallel": 5,
-  "before_tests": "yarn run build",
   "launch_in_ci":  ["Chrome"],
   "launch_in_dev":  ["Chrome", "Firefox", "Safari"],
   "browser_args": {