diff --git a/src/formatter.cr b/src/formatter.cr
index 7c0bf44..b9f8381 100644
--- a/src/formatter.cr
+++ b/src/formatter.cr
@@ -12,7 +12,7 @@ module Tartrazine
     property theme : Theme = Tartrazine.theme("default-dark")
 
     # Format the text using the given lexer.
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       raise Exception.new("Not implemented")
     end
 
diff --git a/src/formatters/ansi.cr b/src/formatters/ansi.cr
index fb3e5c5..19d72f3 100644
--- a/src/formatters/ansi.cr
+++ b/src/formatters/ansi.cr
@@ -11,7 +11,7 @@ module Tartrazine
       "#{i + 1}".rjust(4).ljust(5)
     end
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       tokenizer = Tokenizer.new(lexer, text)
       i = 0
diff --git a/src/formatters/html.cr b/src/formatters/html.cr
index cb33d89..3938820 100644
--- a/src/formatters/html.cr
+++ b/src/formatters/html.cr
@@ -34,7 +34,7 @@ module Tartrazine
                    @weight_of_bold : Int32 = 600)
     end
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       pre, post = wrap_standalone
       outp << pre if standalone?
diff --git a/src/formatters/json.cr b/src/formatters/json.cr
index 857723b..39271c8 100644
--- a/src/formatters/json.cr
+++ b/src/formatters/json.cr
@@ -4,7 +4,7 @@ module Tartrazine
   class Json < Formatter
     property name = "json"
 
-    def format(text : String, lexer : Lexer, io : IO?) : String?
+    def format(text : String, lexer : Lexer, io : IO? = nil) : String?
       outp = io.nil? ? String::Builder.new("") : io
       tokenizer = Tokenizer.new(lexer, text)
       outp << Tartrazine::Lexer.collapse_tokens(tokenizer.to_a).to_json