Bull Call Spread Margin Risk

Hey there,

What Happens, If you buy a Bull Call Spread (P&L is from -1800 to +250) for SPY, but you get assigned and have to buy the 500 Shares for a Price of 603 for example (you only have a margin of 130k$). Does IBKR sell the Shares immediately and you only have to pay the difference (the 1800$) or do you get assigned with the Shares and have to sell them yourself and pay a Charge for the bigger margin?

Thanks for your Help!