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!