It is not a syntax error to use undefined variables in REXX. Undefined variables are defined to have their own name translated to uppercase as their value. As a result it is often difficult to find programming errors that are a result of using undefined variables.

Some tips:

• Add a SIGNAL ON NOVALUE statement to the main section of your programs. This will cause the system to issue a syntax error if you use an undefined variable.
• Be careful to include the period when referring to stems. The variables "A" and "A." are unrelated.
• Misspelled commands will often be interpreted as undefined variables. The line:
  
  sy 'hello'
  
  (which should be "say 'hello'") will be interpreted as:
  
  "SY" || " " || 'hello'
  
  and will be sent to the default command host for execution.