looking at the code, this looks indeed much like a bug (unconditionally converting a possible-undef value to an int).
When did this exactly happen? Before the GUI/TUI was shown?
Would you mind starting the installer in debug mode (under Advanced Options), pressing Ctrl+D when the first prompt appears and post the output of grep . /sys/block/*/size, out of curiosity? (Taking a picture of the screen is also fine if you're not using a KVM/BMC).
Edit: Also, does this prevent you from installing? The installation process should continue, even if an error like this occurs.