An auto-shut-in valve in a chemical injection valve is described for operation at a downhole location in a wellbore. The chemical injection valve includes a closure member that is maintained in an open position by fluid pressure in a chemical supply line such that diminishment of the fluid pressure in the chemical supply line automatically causes the closure member to move to a closed position. A first mating surface on the closure member is exposed to the fluid pressure in the chemical supply line when the closure member is in the open position, and defines a portion of an equivalent differential surface to which the fluid pressure is exposed to maintain the closure member in the open position. The first mating surface is isolated from the fluid pressure in the chemical supply line when the closure member is in the closed position.