Let A(·) be a regular function defined on a connected metric space G whose values are mutually commuting essentially Kato operators in a Banach space. Then the spaces $R^{∞}(A(z))$ and $\overline{N^{∞}(A(z))}$ do not depend on z ∈ G. This generalizes results of B. Aupetit and J. Zemánek.