To reproduce:
- activate sql-mode
- call M-x sql-set-product
- select mysql
- write a part of a query, f.ex. select * from
- call M-x sql-end-of-statement

Error: sql-end-of-statement: Wrong type argument: stringp, nil

In GNU Emacs 25.1.4 (x86_64-unknown-linux-gnu)
 of 2016-09-24 built on kubuntu
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:    Ubuntu 16.04.1 LTS

Configured using:
 'configure --with-x-toolkit=no'

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY LIBXML2 FREETYPE XFT
ZLIB X11

Important settings:
  value of $LC_MONETARY: nl_BE.UTF-8
  value of $LC_NUMERIC: nl_BE.UTF-8
  value of $LC_TIME: nl_BE.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: SQL[MySQL]