diff --git a/ranx/data_structures/qrels.py b/ranx/data_structures/qrels.py
index f0d24bf..4ad6ada 100644
--- a/ranx/data_structures/qrels.py
+++ b/ranx/data_structures/qrels.py
@@ -74,7 +74,8 @@ def __init__(self, qrels: Dict[str, Dict[str, int]] = None, name: str = None):
             # Doc IDs
             doc_ids = [list(doc.keys()) for doc in qrels.values()]
             max_len = max(len(y) for x in doc_ids for y in x)
-            dtype = f"<U{max_len}"
+            width = 1 << (max_len - 1).bit_length() if max_len > 1 else 1
+            dtype = f"<U{width}"
             doc_ids = TypedList([np.array(x, dtype=dtype) for x in doc_ids])
 
             # Scores
diff --git a/ranx/data_structures/run.py b/ranx/data_structures/run.py
index 6df514d..31141d5 100644
--- a/ranx/data_structures/run.py
+++ b/ranx/data_structures/run.py
@@ -59,7 +59,8 @@ def __init__(self, run: Dict[str, Dict[str, float]] = None, name: str = None):
             # Doc IDs
             doc_ids = [list(doc.keys()) for doc in run.values()]
             max_len = max(len(y) for x in doc_ids for y in x)
-            dtype = f"<U{max_len}"
+            width = 1 << (max_len - 1).bit_length() if max_len > 1 else 1
+            dtype = f"<U{width}"
             doc_ids = TypedList([np.array(x, dtype=dtype) for x in doc_ids])
 
             # Scores