Elixir Nerves -- wrong I2C?

Hi! I am very new to all of this but I’m hoping you can help me.

I have an original Raspberry Pi with some sensors attached (GPIO, I2C, SPI). In Raspbian, everything seems to work as intended. I run i2cdetect -y 0 and I see devices at 0x39 and 0x48. I see /dev/i2c-0.

However, I am having trouble exploring with Elixir Circuits Quickstart. If I run Circuits.I2C.bus_names() I get back ["i2c-11"], and Circuits.I2C.detect_devices() just loops i2c-bcm2835 20205000.i2c: i2c transfer timed out.

Any advice?