diff --git a/lib/mutant/cli.rb b/lib/mutant/cli.rb
index 6d766cc7..6e1f8659 100644
--- a/lib/mutant/cli.rb
+++ b/lib/mutant/cli.rb
@@ -206,6 +206,8 @@ module Mutant
     #
     # @param [Enumerable<String>] patterns
     #
+    # @return [undefined]
+    #
     # @api private
     #
     def parse_matchers(patterns)
@@ -219,6 +221,8 @@ module Mutant
     #
     # @param [Object]
     #
+    # @return [undefined]
+    #
     # @api private
     #
     def add_strategies(opts)
@@ -235,6 +239,8 @@ module Mutant
     #
     # @param [Object]
     #
+    # @return [undefined]
+    #
     # @api private
     #
     def add_options(opts)
diff --git a/lib/mutant/reporter/cli/printer/subject.rb b/lib/mutant/reporter/cli/printer/subject.rb
index d739d353..967464f9 100644
--- a/lib/mutant/reporter/cli/printer/subject.rb
+++ b/lib/mutant/reporter/cli/printer/subject.rb
@@ -124,6 +124,12 @@ module Mutant
             # Detailed subject printer
             class Details < self
 
+              # Run subject details printer
+              #
+              # @return [undefined]
+              #
+              # @api private
+              #
               def run
                 puts(subject.identification)
                 object.failed_mutations.each do |mutation|