diff --git a/Gemfile b/Gemfile
index ed45a1f3..9949659c 100644
--- a/Gemfile
+++ b/Gemfile
@@ -7,7 +7,7 @@ gemspec
 group :development do
   gem 'rake', '>= 13'
   gem 'rspec', '~> 3.13.0'
-  gem 'rubocop', '~> 1.65.0'
+  gem 'rubocop', '~> 1.66.0'
   gem 'rubocop-performance', '~> 1.21.0'
   gem 'rubocop-rake', '~> 0.6.0'
   gem 'rubocop-rspec', '~> 3.0.1'
diff --git a/lib/sass/value/number.rb b/lib/sass/value/number.rb
index 69bd4161..f3d91ae3 100644
--- a/lib/sass/value/number.rb
+++ b/lib/sass/value/number.rb
@@ -290,7 +290,7 @@ def coerce_or_convert_value(new_numerator_units, new_denominator_units,
 
         compatibility_error = lambda {
           unless other.nil?
-            message = +"#{self} and"
+            message = "#{self} and"
             message << " $#{other_name}:" unless other_name.nil?
             message << " #{other} have incompatible units"
             message << " (one has units and the other doesn't)" if unitless? || other_unitless