We have studied the capacitance between a two-dimensional electron system and a gate of field-effect transistors, produced from three different wafers with a single remotely doped GaAs/AlxGa1-xAs heterojunction. In the vicinity of the Landau-level filling factor v=1/2, the increment of the capacitance relative to its zero-magnetic-field value, δC1/2, was found to be insensitive to the carrier density, and close to the value as if the particles are noninteracting. This observation implies that electron-electron interaction affects the compressibility of the zero-field and composite-fermion metallic states in a very similar manner.