^CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
  Bad ExternalDataCustomScript key '' in URL template:

   ExternalDataCustomScript:///%\(algo\)/%\(hash\)

  The key must be a valid C identifier.
Call Stack \(most recent call first\):
  BadCustom2.cmake:[0-9]+ \(ExternalData_Add_Target\)
  CMakeLists.txt:[0-9]+ \(include\)$
