$.getJSON sending "null" page?

$.getJSON sending "null" page?

I am using this statement in a Javascript page to send an Ajax request to a PHP page:

$.getJSON("respond4.php",myvar,returnfunction);

"respond4.php" is the PHP page that I am trying to send the variable "myvar" through to. "respond4.php" is in the same directory as the Javascript page that contains the above statement. Yet, I am getting this error in my Apache error log:

"File does not exist: C:/Program Files/Apache Software Foundation/Apache2.2/htdocs/null, referer: http://localhost/newajaxtest01.htm "

Both the source page "newajaxtest01.htm" and the destination page "respond4.php" are located in the directory specified in the error message.

Why does the error indicate that the Ajax statement is trying to send to "null" when "respond4.php" is specified? Is the destination page not getting thru somehow? Or is this an Apache or PHP configuration problem?

Any help would be appreciated.