diff --git a/src/identities/IdentitiesManager.ts b/src/identities/IdentitiesManager.ts
index 201ee93eb5..c5aa731b6e 100644
--- a/src/identities/IdentitiesManager.ts
+++ b/src/identities/IdentitiesManager.ts
@@ -206,7 +206,15 @@ class IdentitiesManager {
       );
     }
     const providerTokens = await this.getTokens(providerId);
-    providerTokens[identityId] = providerToken;
+    // This has to be done in case the key is `__proto__`.
+    // Otherwise, the object will not be correctly serialized.
+    // https://github.com/MatrixAI/Polykey/issues/608
+    Object.defineProperty(providerTokens, identityId, {
+      value: providerToken,
+      writable: true,
+      enumerable: true,
+      configurable: true,
+    });
     const providerIdPath = [
       ...this.identitiesTokensDbPath,
       providerId,