Trouble with Nexus 6p (128G) sideload

Phone stuck on 7.1.1 from December 2016
Trying to avoid unlocking bootloader.

Following several (including excellent androidcentral) guide, adb sideload fails with:
opening 'angler-ota-n2g47w-135b87b2.zip'...
connecting...
falling back to older sideload method...
terminate called after throwing an instance of 'std::bad_alloc'
what(): std::bad_alloc

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

I believe I have properly set developer options, enabled USB debugging, allowed OEM unlocking, and installed current drivers and ADB versions.

Any suggestions welcome.