diff --git a/lib/fluent/plugin/prometheus/placeholder_expander.rb b/lib/fluent/plugin/prometheus/placeholder_expander.rb
index 53f5d28..79f205a 100644
--- a/lib/fluent/plugin/prometheus/placeholder_expander.rb
+++ b/lib/fluent/plugin/prometheus/placeholder_expander.rb
@@ -39,6 +39,8 @@ def build(placeholder_values)
         private
 
         def build_tag(tag)
+          return [] if tag.nil?
+
           tags = tag.split('.')
 
           placeholders = { '${tag}' => tag }