diff --git a/frappe/apps.py b/frappe/apps.py
index 0dd30b4340..fdbb5c74f3 100644
--- a/frappe/apps.py
+++ b/frappe/apps.py
@@ -53,6 +53,8 @@ def get_apps():
 
 
 def get_route(app_name):
+	if app_name not in frappe.get_installed_apps():
+		return "/apps"  # Invalid defaults
 	apps = frappe.get_hooks("add_to_apps_screen", app_name=app_name)
 	app = next((app for app in apps if app.get("name") == app_name), None)
 	return app.get("route") if app and app.get("route") else "/apps"
@@ -89,6 +91,9 @@ def get_default_path():
 
 @frappe.whitelist()
 def set_app_as_default(app_name: str):
+	if app_name not in frappe.get_installed_apps():
+		frappe.throw(_("App {} is not installed").format(frappe.bold(app_name)))
+
 	if frappe.db.get_value("User", frappe.session.user, "default_app") == app_name:
 		frappe.db.set_value("User", frappe.session.user, "default_app", "")
 	else:
diff --git a/frappe/core/doctype/user/test_user.py b/frappe/core/doctype/user/test_user.py
index a091eeff2e..f6ee4ac838 100644
--- a/frappe/core/doctype/user/test_user.py
+++ b/frappe/core/doctype/user/test_user.py
@@ -466,6 +466,21 @@ class TestUser(IntegrationTestCase):
 			sorted(m.get("module_name") for m in get_modules_from_all_apps()),
 		)
 
+	def test_default_app(self):
+		from frappe.apps import get_default_path
+
+		with test_user(roles=["System Manager"]) as user:
+			user.default_app = "next_erp"
+			user.save()
+			self.assertFalse(user.default_app)
+
+			frappe.set_user(user.name)
+			user.db_set("default_app", "next_erp")
+			user.reload()
+			self.assertTrue(user.default_app)
+
+			get_default_path()  # defaults will also trigger hooks logic
+
 	@IntegrationTestCase.change_settings("System Settings", reset_password_link_expiry_duration=1)
 	def test_reset_password_link_expiry(self):
 		new_password = "new_password"
diff --git a/frappe/core/doctype/user/user.py b/frappe/core/doctype/user/user.py
index d62c68dc82..2e5adfb0f9 100644
--- a/frappe/core/doctype/user/user.py
+++ b/frappe/core/doctype/user/user.py
@@ -248,6 +248,9 @@ class User(Document):
 		if self.language == "Loading...":
 			self.language = None
 
+		if self.default_app and self.default_app not in frappe.get_installed_apps():
+			self.default_app = ""
+
 		if (self.name not in ["Administrator", "Guest"]) and (not self.get_social_login_userid("frappe")):
 			self.set_social_login_userid("frappe", frappe.generate_hash(length=39))