OK, I see. Yes, some SEP-IRA custodians allow regular traditional IRA contributions to a SEP-IRA account and some do not. These two types of contributions are reported differently on Forms 5498, so to keep the reporting obvious some custodians simply prohibit regular traditional IRA contributions from being made to a SEP-IRA. My own suggestion is to make SEP and non-SEP contributions to different IRAs for exactly this reason, since doing so makes it difficult for the custodian to mistake one type of contribution to another.
