diff --git a/src/abstract/hash-to-curve.ts b/src/abstract/hash-to-curve.ts
index 2900176..d18a5a5 100644
--- a/src/abstract/hash-to-curve.ts
+++ b/src/abstract/hash-to-curve.ts
@@ -17,7 +17,7 @@ export type Opts = {
   p: bigint;
   m: number;
   k: number;
-  expand?: 'xmd' | 'xof';
+  expand: 'xmd' | 'xof';
   hash: CHash;
 };
 
@@ -145,10 +145,10 @@ export function hash_to_field(msg: Uint8Array, count: number, options: Opts): bi
     prb = expand_message_xmd(msg, DST, len_in_bytes, hash);
   } else if (expand === 'xof') {
     prb = expand_message_xof(msg, DST, len_in_bytes, k, hash);
-  } else if (expand === undefined) {
+  } else if (expand === '_internal_pass') { // for internal tests only
     prb = msg;
   } else {
-    throw new Error('expand must be "xmd", "xof" or undefined');
+    throw new Error('expand must be "xmd" or "xof"');
   }
   const u = new Array(count);
   for (let i = 0; i < count; i++) {
diff --git a/test/bls12-381.test.js b/test/bls12-381.test.js
index 8e699c4..9b19853 100644
--- a/test/bls12-381.test.js
+++ b/test/bls12-381.test.js
@@ -858,7 +858,7 @@ describe('bls12-381/basic', () => {
     const options = {
       p: bls.params.r,
       m: 1,
-      expand: undefined,
+      expand: '_internal_pass',
     };
     for (let vector of SCALAR_VECTORS) {
       const [okmAscii, expectedHex] = vector;