diff --git a/ts/lib/shadow-dom.d.ts b/ts/lib/shadow-dom.d.ts
new file mode 100644
index 000000000..45ba96990
--- /dev/null
+++ b/ts/lib/shadow-dom.d.ts
@@ -0,0 +1,7 @@
+export {};
+
+declare global {
+    interface DocumentOrShadowRoot {
+        getSelection(): Selection | null;
+    }
+}
diff --git a/ts/reviewer/index.ts b/ts/reviewer/index.ts
index 7381a865c..3b69aafa0 100644
--- a/ts/reviewer/index.ts
+++ b/ts/reviewer/index.ts
@@ -80,9 +80,14 @@ function setInnerHTML(element: Element, html: string): void {
 
 const renderError =
     (type: string) =>
-    (error: Error): string => {
+    (error: unknown): string => {
         const errorMessage = String(error).substring(0, 2000);
-        const errorStack = String(error.stack).substring(0, 2000);
+        let errorStack: string;
+        if (error instanceof Error) {
+            errorStack = String(error.stack).substring(0, 2000);
+        } else {
+            errorStack = "";
+        }
         return `<div>Invalid ${type} on card: ${errorMessage}\n${errorStack}</div>`.replace(
             /\n/g,
             "<br>"