diff --git a/src/ameba/config.cr b/src/ameba/config.cr
index 6110c0fc7..0325462c5 100644
--- a/src/ameba/config.cr
+++ b/src/ameba/config.cr
@@ -87,7 +87,7 @@ class Ameba::Config
   property? autocorrect = false
 
   # Returns a filename if reading source file from STDIN.
-  property stdin_filename : String? = nil
+  property stdin_filename : String?
 
   @rule_groups : Hash(String, Array(Rule::Base))
 
diff --git a/src/ameba/formatter/base_formatter.cr b/src/ameba/formatter/base_formatter.cr
index a6ceafdf0..43364e07d 100644
--- a/src/ameba/formatter/base_formatter.cr
+++ b/src/ameba/formatter/base_formatter.cr
@@ -19,10 +19,14 @@ module Ameba::Formatter
 
     # Callback that indicates when source inspection is started.
     # A corresponding source is passed as an argument.
+    #
+    # WARNING: This method needs to be MT safe
     def source_started(source : Source); end
 
     # Callback that indicates when source inspection is finished.
     # A corresponding source is passed as an argument.
+    #
+    # WARNING: This method needs to be MT safe
     def source_finished(source : Source); end
 
     # Callback that indicates when inspection is finished.
diff --git a/src/ameba/formatter/dot_formatter.cr b/src/ameba/formatter/dot_formatter.cr
index 97982622f..22eafc527 100644
--- a/src/ameba/formatter/dot_formatter.cr
+++ b/src/ameba/formatter/dot_formatter.cr
@@ -7,7 +7,7 @@ module Ameba::Formatter
     include Util
 
     @started_at : Time::Span?
-    @mutex = Thread::Mutex.new
+    @mutex = Mutex.new
 
     # Reports a message when inspection is started.
     def started(sources)
diff --git a/src/ameba/formatter/todo_formatter.cr b/src/ameba/formatter/todo_formatter.cr
index 00a2d28f1..ce8d58080 100644
--- a/src/ameba/formatter/todo_formatter.cr
+++ b/src/ameba/formatter/todo_formatter.cr
@@ -3,7 +3,9 @@ module Ameba::Formatter
   # Basically, it takes all issues reported and disables corresponding rules
   # or excludes failed sources from these rules.
   class TODOFormatter < DotFormatter
-    def initialize(@output = STDOUT, @config_path : Path = Config::DEFAULT_PATH)
+    @config_path : Path
+
+    def initialize(@output = STDOUT, @config_path = Config::DEFAULT_PATH)
     end
 
     def finished(sources)