From c061e6a006e26a3473841ca051abdbd63a0b51b1 Mon Sep 17 00:00:00 2001
From: Deepak Cherian <deepak@cherian.net>
Date: Fri, 21 Jun 2024 15:42:54 -0600
Subject: [PATCH] fix types.

---
 xarray/core/dataarray.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xarray/core/dataarray.py b/xarray/core/dataarray.py
index e6ca2ef773b..bd2781e8c76 100644
--- a/xarray/core/dataarray.py
+++ b/xarray/core/dataarray.py
@@ -6772,14 +6772,18 @@ def groupby(
 
         _validate_groupby_squeeze(squeeze)
 
+        grouper: Grouper
         if group is not None:
-            assert not groupers
+            if not groupers:
+                raise ValueError(
+                    "Providing a combination of `group` and **groupers is not supported."
+                )
             grouper = UniqueGrouper()
         else:
             if len(groupers) > 1:
                 raise ValueError("grouping by multiple variables is not supported yet.")
             if not groupers:
-                raise ValueError
+                raise ValueError("**groupers must be provided if `group` is not.")
             group, grouper = next(iter(groupers.items()))
 
         rgrouper = ResolvedGrouper(grouper, group, self)