diff --git a/lib/repubmark/elems/footnote.rb b/lib/repubmark/elems/footnote.rb
index 015a7a4..c79bf79 100644
--- a/lib/repubmark/elems/footnote.rb
+++ b/lib/repubmark/elems/footnote.rb
@@ -42,7 +42,7 @@ module Repubmark
           result += link_text
         end
         result += "\n"
-        result += @caption.to_html
+        result += @caption.to_html.to_s
         if alt_urls&.any?
           result += %[(#{
             alt_urls.map do |alt_url|