diff --git a/Library/Homebrew/test/utils/inreplace_spec.rb b/Library/Homebrew/test/utils/inreplace_spec.rb
index 4283323f2a..20c91672d9 100644
--- a/Library/Homebrew/test/utils/inreplace_spec.rb
+++ b/Library/Homebrew/test/utils/inreplace_spec.rb
@@ -57,7 +57,7 @@ describe Utils::Inreplace do
     it "substitutes pathname within file" do
       # For a specific instance of this, see https://github.com/Homebrew/homebrew-core/blob/a8b0b10/Formula/loki.rb#L48
       described_class.inreplace(file.path) do |s|
-        s.gsub!(Pathname("b"), "f")
+        s.gsub!(Pathname("b"), Pathname("f"))
       end
       expect(File.binread(file)).to eq <<~EOS
         a
diff --git a/Library/Homebrew/utils/string_inreplace_extension.rb b/Library/Homebrew/utils/string_inreplace_extension.rb
index 92fc0ecb43..4c4d4917d4 100644
--- a/Library/Homebrew/utils/string_inreplace_extension.rb
+++ b/Library/Homebrew/utils/string_inreplace_extension.rb
@@ -31,12 +31,12 @@ class StringInreplaceExtension
   #
   # @api public
   sig {
-    params(before: T.any(Pathname, Regexp, String), after: String, audit_result: T::Boolean)
+    params(before: T.any(Pathname, Regexp, String), after: T.any(Pathname, String), audit_result: T::Boolean)
       .returns(T.nilable(String))
   }
   def gsub!(before, after, audit_result = true) # rubocop:disable Style/OptionalBooleanParameter
     before = before.to_s if before.is_a?(Pathname)
-    result = inreplace_string.gsub!(before, after)
+    result = inreplace_string.gsub!(before, after.to_s)
     errors << "expected replacement of #{before.inspect} with #{after.inspect}" if audit_result && result.nil?
     result
   end