diff --git a/returnn/datasets/lm.py b/returnn/datasets/lm.py
index af5a92ec3..9988bef92 100644
--- a/returnn/datasets/lm.py
+++ b/returnn/datasets/lm.py
@@ -67,6 +67,7 @@ def __init__(
         error_on_invalid_seq=True,
         add_delayed_seq_data=False,
         delayed_seq_data_start_symbol="[START]",
+        dtype: Optional[str] = None,
         **kwargs,
     ):
         """
@@ -117,6 +118,7 @@ def __init__(
         :param bool add_delayed_seq_data: will add another data-key "delayed" which will have the sequence.
           delayed_seq_data_start_symbol + original_sequence[:-1].
         :param str delayed_seq_data_start_symbol: used for add_delayed_seq_data.
+        :param dtype: explicit dtype. if not given, automatically determined based on the number of labels.
         """
         super(LmDataset, self).__init__(**kwargs)
 
@@ -245,7 +247,9 @@ def __init__(
             assert not orth_replace_map_file
 
         num_labels = len(self.labels["data"])
-        if num_labels <= 2**7:
+        if dtype:
+            self.dtype = dtype
+        elif num_labels <= 2**7:
             self.dtype = "int8"
         elif num_labels <= 2**8:
             self.dtype = "uint8"