diff --git a/CHANGELOG.rdoc b/CHANGELOG.rdoc
index 4727518dd..bda4d6e67 100644
--- a/CHANGELOG.rdoc
+++ b/CHANGELOG.rdoc
@@ -24,6 +24,7 @@ Bug Fixes:
 
 * Regression: Preserve message-level charset when adding parts (related to Rails ActionMailer) @shields
 * Regression: Adding a part should not reset the mail's charset to nil @railsbob
+* Fix exception when multibyte UTF-8 chars are in multipart boundary @pdg137
 
 Performance:
 
diff --git a/lib/mail/body.rb b/lib/mail/body.rb
index ca763c416..c7ae4bb4a 100644
--- a/lib/mail/body.rb
+++ b/lib/mail/body.rb
@@ -269,13 +269,20 @@ def default_encoding
 
     # split parts by boundary, ignore first part if empty, append final part when closing boundary was missing
     def extract_parts
+      clean_boundary = boundary || ""
+      if raw_source.encoding == Encoding::ASCII_8BIT &&
+         clean_boundary.encoding == Encoding::UTF_8
+        clean_boundary.force_encoding(Encoding::ASCII_8BIT)
+      end
+      clean_boundary = Regexp.escape(boundary || "")
+
       parts_regex = /
         (?:                    # non-capturing group
           \A                |  # start of string OR
           \r?\n                # line break with optional CR
          )
         (
-          --#{Regexp.escape(boundary || "")}  # boundary delimiter
+          --#{clean_boundary}  # boundary delimiter
           (?:--)?                             # with non-capturing optional closing
         )
         (?=\s*$)                              # lookahead matching zero or more spaces followed by line-ending
diff --git a/spec/fixtures/emails/error_emails/utf8_multipart_boundary.eml b/spec/fixtures/emails/error_emails/utf8_multipart_boundary.eml
new file mode 100644
index 000000000..39417251a
--- /dev/null
+++ b/spec/fixtures/emails/error_emails/utf8_multipart_boundary.eml
@@ -0,0 +1,7 @@
+Content-Type: multipart/alternative; boundary="bad-char-µ"
+
+--bad-char-µ
+Content-Type: text/html; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Hello, world!
diff --git a/spec/mail/example_emails_spec.rb b/spec/mail/example_emails_spec.rb
index a2559bdc8..c74642106 100644
--- a/spec/mail/example_emails_spec.rb
+++ b/spec/mail/example_emails_spec.rb
@@ -392,5 +392,10 @@
         expect(mail.encoded).to include("Subject: =?UTF-8?Q?Forma=E7=E3o_FrenetikPolis:_Mega_Campanha_Final?=\r\n =?UTF-8?Q?_Ver=E3o_|_Cursos_de_Setembro?=")
       end
     end
+
+    it 'does not fail on UTF-8 in multipart boundary' do
+      mail = read_fixture('emails', 'error_emails', 'utf8_multipart_boundary.eml')
+      expect(mail.parts.length).to eq(1)
+    end
   end
 end