Invalid parameter values will return an error detailing which parameter is problematic.
Details
The Pareto distribution with rate parameter \(\lambda\) as well as shape
parameter \(\alpha\) has density: $$f\left(x\right) = \frac{\alpha%
\lambda^{\alpha}}{(\lambda + x)^{\alpha + 1}}$$
for \(x \in \mathcal{R}^+\), \(\alpha, \lambda > 0\).