Invalid parameter values will return an error detailing which parameter is problematic.
Details
The Exponential distribution with rate parameter \(\beta\) has
density: $$f\left(x\right) = \frac{1}{\beta}%
\textrm{e}^{-\beta x}$$ for
\(x \in \mathcal{R}^+\), \(\beta > 0\).