diff --git a/src/UrlParams.ts b/src/UrlParams.ts
index 7090019..01ea548 100644
--- a/src/UrlParams.ts
+++ b/src/UrlParams.ts
@@ -35,12 +35,14 @@ export interface UrlParams {
   isPtt: boolean;
   // Whether to use end-to-end encryption
   e2eEnabled: boolean;
-  // The user's ID (only used in Matroska mode)
+  // The user's ID (only used in matryoshka mode)
   userId: string | null;
   // The display name to use for auto-registration
   displayName: string | null;
-  // The device's ID (only used in Matroska mode)
+  // The device's ID (only used in matryoshka mode)
   deviceId: string | null;
+  // The base URL of the homeserver to use for media lookups in matryoshka mode
+  baseUrl: string | null;
   // The BCP 47 code of the language the app should use
   lang: string | null;
 }
@@ -92,6 +94,7 @@ export const getUrlParams = (
     userId: getParam("userId"),
     displayName: getParam("displayName"),
     deviceId: getParam("deviceId"),
+    baseUrl: getParam("baseUrl"),
     lang: getParam("lang"),
   };
 };
diff --git a/src/widget.ts b/src/widget.ts
index b42a2d8..56d83c0 100644
--- a/src/widget.ts
+++ b/src/widget.ts
@@ -80,10 +80,11 @@ export const widget: WidgetHelpers | null = (() => {
       // We need to do this now rather than later because it has capabilities to
       // request, and is responsible for starting the transport (should it be?)
 
-      const { roomId, userId, deviceId } = getUrlParams();
+      const { roomId, userId, deviceId, baseUrl } = getUrlParams();
       if (!roomId) throw new Error("Room ID must be supplied");
       if (!userId) throw new Error("User ID must be supplied");
       if (!deviceId) throw new Error("Device ID must be supplied");
+      if (!baseUrl) throw new Error("Base URL must be supplied");
 
       // These are all the event types the app uses
       const sendState = [
@@ -119,7 +120,7 @@ export const widget: WidgetHelpers | null = (() => {
         },
         roomId,
         {
-          baseUrl: "",
+          baseUrl,
           userId,
           deviceId,
           timelineSupport: true,