"Operator/Operand Mismatch" error while using MatrixOP

Hello. 

I have a question.

 

Why this:

MatrixOP/O volume = 4/3*pi*((d50/2)*3)

compiles,

while this: 
 

MatrixOP/O volume = 4/3*pi*((d50/2)^3)

doesn't?

"3" is highlighted in the error. 

d50 is a one-column 2DWave. 

thank you for your help. 

from DisplayHelpTopic "MatrixOp", functions section:

 

powR(x,y)    Returns x^y  for real x  and y.