4096 CI_DB_mysql_result could not be converted to string

[eluser]Unknown[/eluser]
ERROR: 4096
MESSAGE: 4096 CI_DB_mysql_result could not be converted to string
LINE = '**'

CODE:

Code:

$varX = $this->input->post('param1', TRUE);
$varY = $this->input->post('param2', TRUE);
$varW = $this->functionA->getValue($varX);

**$varZ = $varW . $varY;          //line referenced in the error as the problem

$this->db->where('colX', $varX);
$this->db->where('colY', $varZ);
$query = $this->db->get('table1');

I'm sure there is a simple solution that I am missing here and after reading the:
..input->post
..forums
..CI user guide
..php concatenation, etc...

I still haven't found the answer and I'm sure it has something to do with the variable reference or retrieval.

I'm new to CI, thanks for the help.