Illegal Connection with GPIO Intel® FPGA IP output buffer.

I am using the GPIO Intel® FPGA IP core to connect a Intel FPGA Avalon I2C (Master) Core to the pins of my FPGA.

When I connect the "dout" port of the buffer in bidirectional mode to my core logic I get the following error:
Error(17046): Illegal connection found on I/O output buffer primitive i_od_bi_buf|gpio_0|core|i_loop[1].altera_gpio_bit_i|output_buffer.obuf to i_od_bi_buf|gpio_0|core|i_loop[1].altera_gpio_bit_i|input_buffer.ibuf. The IO output buffer should only drive out to a top-level pin.
If I leave this pin "open" the error goes away; however, this is not suitable as I need to feed the dout port into my I2C master.

I am following the guidance in both user guides yet cannot get this to work.
I'm using Quartus Pro 19.4.0 targetting a Cyclone 10 GX device.