$ emacs --version
GNU Emacs 26.1
$ sbcl --version
SBCL 1.4.13
$ emacs -Q -L <helm directory> -L <helm-core directory> -L <async directory> -L <popup directory> -L <sly directory> -l helm-config -l sly-autoloads --eval '(setq inferior-lisp-program "sbcl")' -f sly

This should start emacs with helm and sly loaded and a repl waiting. If I eval this elisp snippet:
(helm-comp-read "foo " (sly--completion-function-wrapper sly-complete-symbol-function))
I get a helm completions buffer with prompt foo , so far so good. If, however, I start typing into that buffer it will take one character and then quit. Running this in IELM yields the following error:
*** Eval error *** Quit during evaluation
Certain characters do not trigger this, they are: !@#$^&*()_=+{}[]|\,.<>?~; and the space and backquote characters.
Unless I missed one, all the other characters I have tried (U.S qwerty keyboard) result in an eval error. Typing one of the exempt characters and then typeing erroring characters is permitted, such as &rest.
I do not think it is something originating in the returned sly completion function based on some debugging, but I am not certain. I do not recall being able to replicate the error with different collection arguments to helm-comp-read.

I'm working through the backtrace and call tree trying to find out what's up, but would appreciate any help and/or input you can offer. Thanks for all your work with Sly, I really appreciate what it adds to my programming experience.