# What would Kp be if there is no gas reactant

What would $K_\text{p}$ be if there is no gas reactant but the reaction produces gas. I found something confusing: $$K_\text{p}=\frac{P_\text{gas product}}{P_\text{gas reactant}}$$ If there is no gas in the reactant side, then the partial pressure of the reactant gas is 0 so the $K_\text{p}$ would be infinity.

= (P.Pressure of gaseous productA)$^a$ (P.Pressure of gaseous productB)$^b...$