diff --git a/sc2ts/debug.py b/sc2ts/debug.py
index 468f20d..e109a0e 100644
--- a/sc2ts/debug.py
+++ b/sc2ts/debug.py
@@ -458,6 +458,7 @@ def _preprocess_nodes(self, show_progress):
             total=ts.num_nodes,
             disable=not show_progress,
         )
+        group_id_prefix_map = {}
         for node in iterator:
             md = node.metadata
             self.nodes_metadata[node.id] = md
@@ -468,6 +469,14 @@ def _preprocess_nodes(self, show_progress):
             if group_id is not None:
                 # Shorten key for readability.
                 gid = group_id[: self.sample_group_id_prefix_len]
+                if gid not in group_id_prefix_map:
+                    group_id_prefix_map[gid] = group_id
+                if group_id != group_id_prefix_map[gid]:
+                    raise ValueError(
+                        "Sample group ID prefix collision. "
+                        "Increase sample_group_id_prefix_len "
+                        "(default=10)"
+                    )
                 self.sample_group_nodes[gid].append(node.id)
             if node.is_sample():
                 self.nodes_date[node.id] = md["date"]
diff --git a/sc2ts/inference.py b/sc2ts/inference.py
index 687b4a4..a3690c8 100644
--- a/sc2ts/inference.py
+++ b/sc2ts/inference.py
@@ -932,15 +932,15 @@ def __post_init__(self):
 
     @property
     def strains(self):
-        return [s.strain for s in self.samples]
+        return [str(s.strain) for s in self.samples]
 
     @property
     def date_count(self):
-        return collections.Counter([s.date for s in self.samples])
+        return collections.Counter([str(s.date) for s in self.samples])
 
     @property
     def pango_count(self):
-        return collections.Counter([s.pango for s in self.samples])
+        return collections.Counter([str(s.pango) for s in self.samples])
 
     def __len__(self):
         return len(self.samples)