diff --git a/lib/steep/interface/type_param.rb b/lib/steep/interface/type_param.rb
index 4877d47d..34eae8ca 100644
--- a/lib/steep/interface/type_param.rb
+++ b/lib/steep/interface/type_param.rb
@@ -62,7 +62,7 @@ def self.rename(params, conflicting_names = params.map(&:name), new_names = conf
       end
 
       def to_s
-        buf = ""
+        buf = +""
 
         if unchecked
           buf << "unchecked "
diff --git a/lib/steep/server/lsp_formatter.rb b/lib/steep/server/lsp_formatter.rb
index 15b4ce8b..8a1f556d 100644
--- a/lib/steep/server/lsp_formatter.rb
+++ b/lib/steep/server/lsp_formatter.rb
@@ -361,7 +361,7 @@ def name_and_params(name, params)
           "#{name}"
         else
           ps = params.each.map do |param|
-            s = ""
+            s = +""
             if param.unchecked?
               s << "unchecked "
             end
diff --git a/test/cli_test.rb b/test/cli_test.rb
index 201ab653..3cd19fb9 100644
--- a/test/cli_test.rb
+++ b/test/cli_test.rb
@@ -517,7 +517,7 @@ class Person
       pid = spawn(*steep, "watch", "app/lib", out: w, chdir: current_dir.to_s)
       w.close
 
-      output = ""
+      output = +""
 
       begin
         read_thread = Thread.new do
@@ -578,7 +578,7 @@ class Person
       pid = spawn(*steep.push("watch", "app/models/person.rb"), out: w, chdir: current_dir.to_s)
       w.close
 
-      output = ""
+      output = +""
 
       begin
         read_thread = Thread.new do