diff --git a/lib/chupa-text/decomposers/mail.rb b/lib/chupa-text/decomposers/mail.rb
index ef547ba..d3bf108 100644
--- a/lib/chupa-text/decomposers/mail.rb
+++ b/lib/chupa-text/decomposers/mail.rb
@@ -43,7 +43,13 @@ def decompose(data)
         end
         parts.each_with_index do |part, i|
           body = part.body.decoded
-          body.force_encoding(part.charset) if part.charset
+          if part.charset
+            begin
+              body.force_encoding(part.charset)
+            rescue ArgumentError
+              raise UnknownEncodingError.new(data, part.charset)
+            end
+          end
 
           part_data = TextData.new(body, :source_data => data)
           uri = data.uri.dup
diff --git a/test/fixture/unknown-encoding.eml b/test/fixture/unknown-encoding.eml
new file mode 100644
index 0000000..88c9542
--- /dev/null
+++ b/test/fixture/unknown-encoding.eml
@@ -0,0 +1,9 @@
+Mime-Version: 1.0
+Subject: Hello
+From: Sender <from@example.com>
+To: Recipient <to@example.com>
+Date: Sun, 19 Feb 2017 00:27:55 +0900 (JST)
+Content-Type: Text/Plain; charset=iso-2022-jp-3
+Content-Transfer-Encoding: 8bit
+
+World
diff --git a/test/test-mail.rb b/test/test-mail.rb
index 8cab4ee..f6905a2 100644
--- a/test/test-mail.rb
+++ b/test/test-mail.rb
@@ -190,5 +190,18 @@ def decompose
         super(fixture_path("nested-rfc822.eml"))
       end
     end
+
+    sub_test_case("unknown encoding") do
+      def test_body
+        assert_raise(ChupaText::UnknownEncodingError) do
+          decompose
+        end
+      end
+
+      private
+      def decompose
+        super(fixture_path("unknown-encoding.eml"))
+      end
+    end
   end
 end