diff --git a/Library/Homebrew/cask/exceptions.rb b/Library/Homebrew/cask/exceptions.rb
index 401203f8ae..13e04a008a 100644
--- a/Library/Homebrew/cask/exceptions.rb
+++ b/Library/Homebrew/cask/exceptions.rb
@@ -174,7 +174,7 @@ module Cask
     end
 
     def to_s
-      s = "Failed to quarantine #{path}."
+      s = +"Failed to quarantine #{path}."
 
       unless reason.empty?
         s << " Here's the reason:\n"
@@ -188,7 +188,7 @@ module Cask
 
   class CaskQuarantinePropagationError < CaskQuarantineError
     def to_s
-      s = "Failed to quarantine one or more files within #{path}."
+      s = +"Failed to quarantine one or more files within #{path}."
 
       unless reason.empty?
         s << " Here's the reason:\n"
@@ -202,7 +202,7 @@ module Cask
 
   class CaskQuarantineReleaseError < CaskQuarantineError
     def to_s
-      s = "Failed to release #{path} from quarantine."
+      s = +"Failed to release #{path} from quarantine."
 
       unless reason.empty?
         s << " Here's the reason:\n"
diff --git a/Library/Homebrew/cmd/gist-logs.rb b/Library/Homebrew/cmd/gist-logs.rb
index 6986f6160f..b5cc85f0da 100644
--- a/Library/Homebrew/cmd/gist-logs.rb
+++ b/Library/Homebrew/cmd/gist-logs.rb
@@ -82,7 +82,7 @@ module Homebrew
 
   def brief_build_info(f)
     build_time_str = f.logs.ctime.strftime("%Y-%m-%d %H:%M:%S")
-    s = <<~EOS
+    s = +<<~EOS
       Homebrew build logs for #{f.full_name} on #{OS_VERSION}
     EOS
     if args.with_hostname?