PQexecParams and "SET TIME ZONE $1" gets 'syntax error at or near "$1" at character 15'

I get get this same error

syntax error at or near "$1" at character 15

if I feed "const char *command" with the following texts.

SET TIME ZONE $1
SET TIME ZONE $1::TEXT

For some reasons, I can not add quotes around $1 as follows.

SET TIME ZONE '$1'
SET TIME ZONE '$1'::TEXT

Statements like "SELECT ... WHERE $1=..." have been working flawlessly.

It looks like PQexecParams doesn't handle statement with "SET ... $1". 

Are there workarounds for me or PQexecParams needs improvement?

Thank you!