Error due to LOG in specify equations

[krish]

Hi. I am using Tecplot 360 Ex 2017 R1. I am calculating the logarithmic value of the density gradient and create a contour. I have the following expression:

{log rho} = LOG({Density_Gradient})

When I click "Compute", the following error occurs:

"Bad argument to LN.
Equation where error occurred:

{log rho} = LOG({Density_Gradient})"

Does anyone know why this error occurs?

Any help would be appreciated.