diff --git a/flox/core.py b/flox/core.py
index e1e7dc039..4f34c020b 100644
--- a/flox/core.py
+++ b/flox/core.py
@@ -1809,4 +1809,8 @@ def groupby_reduce(
     if _is_minmax_reduction(func) and is_bool_array:
         result = result.astype(bool)
 
+    # we lose dtype information when creating a pandas Index
+    # That's always int64 or float64, so cast back
+    # TODO: migrate to NumericIndex but I think this will just fix itself
+    groups = tuple(g.astype(b.dtype) for g, b in zip(groups, bys))
     return (result, *groups)