Trying to get Nucleo-F042K6 into DFU mode

I'm developing an application for the F042 series chips. I'm using Nucleo-F042K6 for development.

I want to allow users to flush firmware using DFU mode. I am unclear on how to force the device into the DFU bootloader. Some material I read suggests that the board is wired so that I only need to connect USB to PA10, PA11, VIN and GND and it goes into bootloader. Other sources suggest I need a 1.5k pullup on PA11. I must be missing something very simple. With just the USB connections, nothing happens when I plug USB in. With a pullup to DM, the host attempts enumeration, but gets no response from the chip.

On a Linux host, this the output from dmesg:

[945270.896795] usb 1-9: new full-speed USB device number 84 using xhci_hcd
[945271.024788] usb 1-9: device descriptor read/64, error -71
[945271.260797] usb 1-9: device descriptor read/64, error -71
[945271.496748] usb 1-9: new full-speed USB device number 85 using xhci_hcd
[945271.624734] usb 1-9: device descriptor read/64, error -71
[945271.860727] usb 1-9: device descriptor read/64, error -71
[945271.968716] usb usb1-port9: attempt power cycle
[945272.620662] usb 1-9: new full-speed USB device number 86 using xhci_hcd
[945272.620891] usb 1-9: Device not responding to setup address.
[945272.828796] usb 1-9: Device not responding to setup address.
[945273.036615] usb 1-9: device not accepting address 86, error -71
[945273.164584] usb 1-9: new full-speed USB device number 87 using xhci_hcd
[945273.164776] usb 1-9: Device not responding to setup address.
[945273.372685] usb 1-9: Device not responding to setup address.
[945273.580574] usb 1-9: device not accepting address 87, error -71
[945273.580671] usb usb1-port9: unable to enumerate USB device