diff --git a/megatron/tokenizer/tokenizer.py b/megatron/tokenizer/tokenizer.py
index 348c7cefe..e450504c8 100644
--- a/megatron/tokenizer/tokenizer.py
+++ b/megatron/tokenizer/tokenizer.py
@@ -31,6 +31,8 @@ def build_tokenizer(args):
     """Initialize tokenizer."""
     if args.rank == 0:
         print("> building {} tokenizer ...".format(args.tokenizer_type), flush=True)
+    
+    assert args.tokenizer_type is not None, "tokenizer_type must be specified in the .yml config"
 
     # Select and instantiate the tokenizer.
     if args.tokenizer_type.lower() == "GPT2BPETokenizer".lower():