nimble erroneously split arguments while passing to the nim compiler

[jxy]

$ nimble --verbose --debug c -r -d:myVar="string with spaces" MyApp
  Verifying dependencies for tmp@0.0.0
  Compiling MyApp (from package tmp) using c backend
  Executing "/Users/jin/pkg/src/Nim/bin/nim" c --noNimblePath -r -d:myVar=string with spaces  "MyApp"
Hint: used config file '/Users/jin/pkg/src/Nim-devel/config/nim.cfg' [Conf]
Hint: system [Processing]
Hint: with [Processing]
Error: cannot open 'with.nim'
   Finished with exit code 256
     Error: Execution failed with exit code 256
        ... Command: "/Users/jin/pkg/src/Nim/bin/nim" c --noNimblePath -r -d:myVar=string with spaces  "MyApp"

The "string with spaces" is not preserved. Using the compiler directly works fine.