diff --git a/addons/api/mirage/config.js b/addons/api/mirage/config.js
index cd4de1f31..b7ff24208 100644
--- a/addons/api/mirage/config.js
+++ b/addons/api/mirage/config.js
@@ -632,7 +632,23 @@ function routes() {
       { queryParams: { credential_store_id: credentialStoreId } },
     ) => credentialLibraries.where({ credentialStoreId }),
   );
-  this.get('/credential-libraries/:id');
+  this.get(
+    '/credential-libraries/:id',
+    function ({ credentialLibraries }, { params: { id } }) {
+      const library = credentialLibraries.find(id);
+      // BE API only updates these fields if there's an update mask, otherwise they remain unmodified
+      // but we don't need this flow in our mocks, so we are deleting fields with null value
+      const { credentialMappingOverrides } = library;
+      if (Object.keys(credentialMappingOverrides || {}).length) {
+        Object.keys(credentialMappingOverrides).forEach((key) => {
+          if (credentialMappingOverrides[key] === null) {
+            delete credentialMappingOverrides[key];
+          }
+        });
+      }
+      return library;
+    },
+  );
   this.post(
     '/credential-libraries',
     function ({ credentialStores, credentialLibraries }) {
diff --git a/ui/admin/tests/acceptance/credential-library/update-test.js b/ui/admin/tests/acceptance/credential-library/update-test.js
index 54ef95b86..89f97622f 100644
--- a/ui/admin/tests/acceptance/credential-library/update-test.js
+++ b/ui/admin/tests/acceptance/credential-library/update-test.js
@@ -133,8 +133,6 @@ module('Acceptance | credential-libraries | update', function (hooks) {
     assert.strictEqual(credentialLibrary.attributes.path, 'path');
     assert.deepEqual(credentialLibrary.credentialMappingOverrides, {
       private_key_attribute: 'key',
-      private_key_passphrase_attribute: null,
-      username_attribute: null,
     });
   });