diff --git a/specification/ml/_types/inference.ts b/specification/ml/_types/inference.ts
index 84d8ca8e69..01f5473544 100644
--- a/specification/ml/_types/inference.ts
+++ b/specification/ml/_types/inference.ts
@@ -287,7 +287,7 @@ export class FillMaskInferenceOptions {
   tokenization?: TokenizationConfigContainer
   /** The field that is added to incoming documents to contain the inference prediction. Defaults to predicted_value. */
   results_field?: string
-  vocabulary?: Vocabulary
+  vocabulary: Vocabulary
 }
 
 /** Question answering inference options */