STM32F4 Discovery CubeMX and USB

I am setting up a USB OTG based project on the STM32F4 Discovery Board.  I am using STM32CubeMX 4.23.0 and STM32Cube_FW_F4_v1.18.0.  This is the same set up on my Windows 10 and Linux Mint 18.2 PCs and both have the same error.

The clock configuration tab is always indicating that the clock set up is in error.  In the PLL Source Mux,  the HSI input is selected and has a red box around it with the message 'PLL Mux should have HSE as input selected'.  The HSE button is, of course, grayed out.  It generates the code but I get warnings about an invalid clock configuration.

Is this a problem with STM32CubeMX?  Is there actually an invalid clock configuration?  Is the generated code safe to use?