syntax errors table modifications

I keep getting syntax errors with the simplest things. MS SQL db

ALTER TABLE "dbo"."Members"
DROP PRIMARY KEY;
/* SQL Error (156): Incorrect syntax near the keyword 'PRIMARY'. */

Tried with ' instead of "?

With the latest HeidiSQL version?

I recall I touched that piece of internal SQL code several times in the past, and I was cursing about the myriad of differences between MS SQL versions. Which server version is it on your end?