diff --git a/src/Identity/Models/Response/Accounts/RegisterFinishResponseModel.cs b/src/Identity/Models/Response/Accounts/RegisterFinishResponseModel.cs
index d7c7b94366..564150ab30 100644
--- a/src/Identity/Models/Response/Accounts/RegisterFinishResponseModel.cs
+++ b/src/Identity/Models/Response/Accounts/RegisterFinishResponseModel.cs
@@ -6,5 +6,12 @@ public class RegisterFinishResponseModel : ResponseModel
 {
     public RegisterFinishResponseModel()
         : base("registerFinish")
-    { }
+    {
+        // We are setting this to an empty string so that old mobile clients don't break, as they reqiure a non-null value.
+        // This will be cleaned up in https://bitwarden.atlassian.net/browse/PM-21720.
+        CaptchaBypassToken = string.Empty;
+    }
+
+    public string CaptchaBypassToken { get; set; }
+
 }