Describe the bug
Here's a sample SConstruct which demonstrates the issue.

env=Environment()

if ARGUMENTS.get('dir', 0):
    env.Program('dir1/main/main','main.c')
else:
    env.Program('dir1/main','main.c')

If you run as

scons
# followed by
scons dir=1

Or the reverse

You'll get a stack trace.

Required information

• Link to SCons Users thread discussing your issue.
• Version of SCons
  Any
• Version of Python
  Any
• Which python distribution if applicable (python.org, cygwin, anaconda, macports, brew,etc)
  Any
• How you installed SCons
  N/A
• What Platform are you on? (Linux/Windows and which version)
  N/A
• How to reproduce your issue? Please include a small self contained reproducer. Likely a SConstruct should do for most issues.
  See description above
• How you invoke scons (The command line you're using "scons --flags some_arguments")
  See description above