Adding space to PRIMARY filegroup

It’s always the simple things that catch you out on occasion.
Adding space to SQL Server database primary filegroup returned this error:

Msg 1105, Level 17, State 2, Server WG0140, Line 1
Could not allocate space for object 'sys.sysfiles1'.'sysfiles1' in database 'SITE' because the 'PRIMARY' filegroup is full.

To add space to the primary filegroup requires some free space in the first MDF file.
Adding just 1MB to the MDF file allowed the addition of another (NDF) file without error.