diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index f8b2d12887..d80004daea 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -1139,10 +1139,10 @@ class FormulaInstaller
   end
 
   def forbidden_license_check
-    forbidden_licenses = Homebrew::EnvConfig.forbidden_licenses.to_s
+    forbidden_licenses = Homebrew::EnvConfig.forbidden_licenses.dup
     SPDX::ALLOWED_LICENSE_SYMBOLS.each do |s|
       pattern = /#{s.to_s.tr("_", " ")}/i
-      forbidden_licenses = forbidden_licenses.sub(pattern, s.to_s)
+      forbidden_licenses.sub!(pattern, s.to_s)
     end
     forbidden_licenses = forbidden_licenses.split(" ").to_h do |license|
       [license, SPDX.license_version_info(license)]