diff --git a/turinglab/__main__.py b/turinglab/__main__.py
index a916508..5b4133d 100644
--- a/turinglab/__main__.py
+++ b/turinglab/__main__.py
@@ -32,7 +32,9 @@ def main():
     test_data = []
 
     for test in args.tests:
-        test = test.replace(args.empty_character, 'λ')
+
+        if args.empty_character:
+            test = test.replace(args.empty_character, 'λ')
 
         tm = Emulator(program, test)