diff --git a/packages/core/assemblyManager/assemblyManager.ts b/packages/core/assemblyManager/assemblyManager.ts
index 4fe6f027fb..567d981422 100644
--- a/packages/core/assemblyManager/assemblyManager.ts
+++ b/packages/core/assemblyManager/assemblyManager.ts
@@ -32,7 +32,7 @@ export default function assemblyManagerFactory(
       get assemblyList() {
         return [
           ...getParent(self).jbrowse.assemblies.slice(),
-          ...getParent(self).session.sessionAssemblies.slice(),
+          ...(getParent(self).session.sessionAssemblies?.slice() || []),
         ]
       },
 
diff --git a/products/jbrowse-web/src/__snapshots__/sessionModelFactory.test.js.snap b/products/jbrowse-web/src/__snapshots__/sessionModelFactory.test.js.snap
index 1e3d479e9a..16c6851aba 100644
--- a/products/jbrowse-web/src/__snapshots__/sessionModelFactory.test.js.snap
+++ b/products/jbrowse-web/src/__snapshots__/sessionModelFactory.test.js.snap
@@ -7,6 +7,7 @@ Object {
   "drawerWidth": 384,
   "margin": 0,
   "name": "testSession",
+  "sessionAssemblies": Array [],
   "sessionConnections": Array [],
   "sessionTracks": Array [],
   "views": Array [],
diff --git a/products/jbrowse-web/src/sessionModelFactory.ts b/products/jbrowse-web/src/sessionModelFactory.ts
index 4ebaa38169..23d2c93f05 100644
--- a/products/jbrowse-web/src/sessionModelFactory.ts
+++ b/products/jbrowse-web/src/sessionModelFactory.ts
@@ -42,7 +42,7 @@ declare interface ReferringNode {
 
 export default function sessionModelFactory(
   pluginManager: PluginManager,
-  assemblyConfigSchemasType: any,
+  assemblyConfigSchemasType = types.frozen(), // if not using sessionAssemblies
 ) {
   const minDrawerWidth = 128
   return types