Simple RPN with EXP goes wrong

Hi,

Until now I thought that I understood how RPN works, but apparently not 
this time.

Example: 3*x^2

I thought this would be in rpn: 3,x,2,EXP,*

However, rrdtool is complaining "ERROR: RPN final stack size != 1"; one 
operand would be missing.
Where am I wrong?