This is with an empty init.el, and the following early-init.el:

D:\...\.emacs.d> type early-init.el
(setq default-frame-alist '((left . (+ 0))))

D:\...\.emacs.d> emacs.exe

lisp.h:1231: Emacs fatal error: assertion failed: FIXNUMP (a)

Setting the same value on init.el works.

In GNU Emacs 27.0.50 (build 1, x86_64-w64-mingw32)
of 2019-09-16 built on ODIEFAST
Repository revision: b3e4b50578778e03327b049f7a595981bfbf3713
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 10.0.18362
System Description: Microsoft Windows 10 Home (v10.0.1903.18362.356)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
'configure --prefix=/d/Devel/emacs/repo/trunk --with-modules
--enable-checking=yes
--enable-locallisppath=%emacs_dir%/../site-lisp:%emacs_dir%/share/emacs/@VER@/site-lisp:%emacs_dir%/share/emacs/site-lisp
'CFLAGS=-Og -ggdb3'
PKG_CONFIG_PATH=/mingw64/lib/pkgconfig:/mingw64/share/pkgconfig'