diff --git a/pymc/backends/zarr.py b/pymc/backends/zarr.py
index 27aa5f885c..f64336e11a 100644
--- a/pymc/backends/zarr.py
+++ b/pymc/backends/zarr.py
@@ -305,6 +305,7 @@ def completed_draws_and_divergences(self, chain_specific: bool = False) -> tuple
         """
         # No need to iterate over ZarrChain instances because the zarr group is
         # shared between them
+        idx: int | slice
         if chain_specific:
             idx = self.chain
         else: