WARNING...be careful making starters!

I usually make starters in a 1L erlemneyer flask that has a glass cap that fits within the glossed opening of flask (fitting is by design, not DIY'd). I boil on my oven top with about 900mL starting volume, with the lid/stopper in the opening during the boil, boil for 15-20 minutes, no problems. Today I filled it up to 1000mL, placed the lid on top of the flask and proceeded to start boil. Five minutes into the boil...BOOM...hot/sticky starter sprayed throughout the kitchen. No one was hurt, but quite a mess was made. I'm not sure if it was the extra volume or the flask, either way I'll be sticking to foil to cover the mouth from now on. Be safe out there!