diff --git a/lib/csv.rb b/lib/csv.rb
index 804b941..a42b86a 100644
--- a/lib/csv.rb
+++ b/lib/csv.rb
@@ -235,6 +235,7 @@ class CSV
     #
     def initialize(headers, fields, header_row = false)
       @header_row = header_row
+      headers.each { |h| h.freeze if h.is_a? String }
 
       # handle extra headers or fields
       @row = if headers.size > fields.size
@@ -2205,6 +2206,7 @@ class CSV
       # prepare converted and unconverted copies
       row      = @headers                       if row.nil?
       @headers = convert_fields(@headers, true)
+      @headers.each { |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..22907d2 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_instance_of(String, keys[h])
+          assert_same(header, keys[h])
+        end
+      end
+    end
+  end
+
   def test_write_hash_with_headers_array
     File.unlink(@path)
 
diff --git a/test/csv/test_row.rb b/test/csv/test_row.rb
index 697c7d5..b4ac1c3 100755
--- a/test/csv/test_row.rb
+++ b/test/csv/test_row.rb
@@ -297,7 +297,12 @@ class TestCSV::Row < TestCSV
   end
 
   def test_to_hash
-    assert_equal({"A" => nil, "B" => 2, "C" => 3}, @row.to_hash)
+    hash = @row.to_hash
+    assert_equal({"A" => nil, "B" => 2, "C" => 3}, hash)
+    hash.keys.each_with_index do |string_key, h|
+      assert(string_key.frozen?)
+      assert_same(string_key, @row.headers[h])
+    end
   end
 
   def test_to_csv