# rental cost of capital

I am estimating time series for rental price of capital using Hall-Jorgenson method: $$r=\frac{P_k}{P}(i-inf+\delta)$$ here, $$r$$ is the rental cost of capital, $$P_k$$ is the price of capital goods (price index for capital goods), $$P$$ is a deflator, $$i$$ is nominal interest rate, $$inf$$ is an inflation rate and $$\delta$$ is depreciation rate of physical capital stock.

Because inflation rate is high in some periods, the rental cost of capital is negative for these periods; this does not make sense, any little help is welcome!