CMake can't find Ninja on MacOS

I have a CMake project loaded in Qt Creator on MacOS. It builds fine using the Unix Makefiles generator, but if I change it to Ninja I get the following message:

CMake Error: CMake was unable to find a build program corresponding to "Ninja". CMAKE_MAKE_PROGRAM is not set. You probably need to select a different build tool.

I definitely have Ninja installed and in the PATH and I have restarted Qt Creator. Moreover, the command that Qt Creator tries to run works fine if I copy and paste it into a shell. Also by building on the command line it is possible to get the build directory into a state where Qt will work fine, but then if you clean the project and try to build it again in Qt Creator you get the above error.

I've tried setting CMAKE_MAKE_PROGRAM in various places but it never seems to make a difference.