diff --git a/malariagen_data/anoph/snp_data.py b/malariagen_data/anoph/snp_data.py
index fa2111fa1..3741088cb 100644
--- a/malariagen_data/anoph/snp_data.py
+++ b/malariagen_data/anoph/snp_data.py
@@ -799,7 +799,7 @@ def _locate_site_class(
         try:
             loc_ann = self._cache_locate_site_class[cache_key]
 
-        except KeyError as exc:
+        except KeyError:
             # Access site annotations data.
             ds_ann = self._site_annotations_raw(
                 contig=region.contig,
@@ -923,7 +923,7 @@ def _locate_site_class(
                 ) | ((seq_cls == SEQ_CLS_DOWNSTREAM) & (seq_relpos_start > 10_000))
 
             else:
-                raise NotImplementedError(site_class) from exc
+                raise NotImplementedError(site_class) from None
 
             # N.B., site annotations data are provided for every position in the genome. We need to
             # therefore subset to SNP positions.