diff --git a/asciiWriter/marks.py b/asciiWriter/marks.py
index 6849e36..052e1a0 100644
--- a/asciiWriter/marks.py
+++ b/asciiWriter/marks.py
@@ -21,7 +21,7 @@ def text (mark_text):
     return char
   return f
  
-def single (char):
+def single (char=' '):
   """Mark with a single character."""
   def f():
     return char