diff --git a/lib/axlsx/workbook/worksheet/comment.rb b/lib/axlsx/workbook/worksheet/comment.rb
index e5dbe41c..3f84fba5 100644
--- a/lib/axlsx/workbook/worksheet/comment.rb
+++ b/lib/axlsx/workbook/worksheet/comment.rb
@@ -47,7 +47,7 @@ def vml_shape
     # the comment.
     # @return [Integer]
     def author_index
-      @comments.authors.index(author)
+      @comments.author_index(author)
     end
 
     # @see ref
@@ -61,7 +61,6 @@ def ref=(v)
     # @param [String] str
     # @return [String]
     def to_xml_string(str = "")
-      author = @comments.authors[author_index]
       str << ('<comment ref="' << ref << '" authorId="' << author_index.to_s << '">')
       str << '<text>'
       unless author.to_s == ""
diff --git a/lib/axlsx/workbook/worksheet/comments.rb b/lib/axlsx/workbook/worksheet/comments.rb
index dfc6d143..c45d8c31 100644
--- a/lib/axlsx/workbook/worksheet/comments.rb
+++ b/lib/axlsx/workbook/worksheet/comments.rb
@@ -44,13 +44,21 @@ def add_comment(options={})
       raise ArgumentError, "Comment requires ref" unless options[:ref]
       self << Comment.new(self, options)
       yield last if block_given?
+      clear_author_caches
       last
     end
 
     # A sorted list of the unique authors in the contained comments
     # @return [Array]
     def authors
-      map { |comment| comment.author.to_s }.uniq.sort
+      @authors ||= map { |comment| comment.author.to_s }.uniq.sort
+    end
+
+    # Returns the index of the given author in the sorted authors array
+    # @param [String] author An author
+    # @return [Integer]
+    def author_index(author)
+      (@author_indexes ||= {})[author] ||= authors.index(author)
     end
 
     # The relationships required by this object
@@ -77,6 +85,13 @@ def to_xml_string(str="")
 
     end
 
+    protected
+
+    def clear_author_caches
+      @authors = nil
+      @author_indexes = nil
+    end
+
   end
 
 end