diff --git a/lib/csv.rb b/lib/csv.rb
index 804b941..240f135 100644
--- a/lib/csv.rb
+++ b/lib/csv.rb
@@ -2205,6 +2205,7 @@ class CSV
       # prepare converted and unconverted copies
       row      = @headers                       if row.nil?
       @headers = convert_fields(@headers, true)
+      @headers.map { |h| h.freeze if h.is_a? String }
 
       if @return_headers                                     # return headers
         return self.class::Row.new(@headers, row, true)
diff --git a/test/csv/test_interface.rb b/test/csv/test_interface.rb
index ad310fa..901a313 100755
--- a/test/csv/test_interface.rb
+++ b/test/csv/test_interface.rb
@@ -198,6 +198,25 @@ class TestCSV::Interface < TestCSV
     end
   end
 
+  def test_write_hash_with_string_keys
+    File.unlink(@path)
+
+    lines = [{a: 1, b: 2, c: 3}, {a: 4, b: 5, c: 6}]
+    CSV.open( @path, "wb", headers: true ) do |csv|
+      csv << lines.first.keys
+      lines.each { |line| csv << line }
+    end
+    CSV.open( @path, "rb", headers: true ) do |csv|
+      csv.each do |line|
+        csv.headers.each_with_index do |header, h|
+          keys = line.to_hash.keys
+          assert_equal(String, keys[h].class)
+          assert_equal(header.object_id, keys[h].object_id)
+        end
+      end
+    end
+  end
+
   def test_write_hash_with_headers_array
     File.unlink(@path)