diff --git a/WebCryptoAPI/digest/cshake.tentative.https.any.js b/WebCryptoAPI/digest/cshake.tentative.https.any.js
index d5f790e42b4982..3162383ee9dbf9 100644
--- a/WebCryptoAPI/digest/cshake.tentative.https.any.js
+++ b/WebCryptoAPI/digest/cshake.tentative.https.any.js
@@ -195,16 +195,17 @@ Object.keys(digestedData).forEach(function (alg) {
 
         promise_test(function (test) {
           var buffer = new Uint8Array(sourceData[size]);
-          return crypto.subtle
+          var promise = crypto.subtle
             .digest({ name: alg, length: length }, buffer)
             .then(function (result) {
-              // Alter the buffer after calling digest
-              buffer[0] = ~buffer[0];
               assert_true(
                 equalBuffers(result, digestedData[alg][length][size]),
                 'digest matches expected'
               );
             });
+          // Alter the buffer after calling digest
+          buffer[0] = ~buffer[0];
+          return promise;
         }, alg + ' with ' + length + ' bit output and ' + size + ' source data and altered buffer after call');
       }
     });
diff --git a/WebCryptoAPI/digest/sha3.tentative.https.any.js b/WebCryptoAPI/digest/sha3.tentative.https.any.js
index fc33608e07ab14..0842a7d50d63f7 100644
--- a/WebCryptoAPI/digest/sha3.tentative.https.any.js
+++ b/WebCryptoAPI/digest/sha3.tentative.https.any.js
@@ -132,14 +132,15 @@ Object.keys(sourceData).forEach(function (size) {
 
       promise_test(function (test) {
         var buffer = new Uint8Array(sourceData[size]);
-        return crypto.subtle.digest(alg, buffer).then(function (result) {
-          // Alter the buffer after calling digest
-          buffer[0] = ~buffer[0];
+        var promise = crypto.subtle.digest(alg, buffer).then(function (result) {
           assert_true(
             equalBuffers(result, digestedData[alg][size]),
             'digest matches expected'
           );
         });
+        // Alter the buffer after calling digest
+        buffer[0] = ~buffer[0];
+        return promise;
       }, alg + ' with ' + size + ' source data and altered buffer after call');
     }
   });