diff --git a/PIL/PdfImagePlugin.py b/PIL/PdfImagePlugin.py
index 725f22ecf..2a8a8d443 100644
--- a/PIL/PdfImagePlugin.py
+++ b/PIL/PdfImagePlugin.py
@@ -144,7 +144,7 @@ def _save(im, fp, filename):
         if bits == 1:
             # FIXME: the hex encoder doesn't support packed 1-bit
             # images; do things the hard way...
-            data = im.tostring("raw", "1")
+            data = im.tobytes("raw", "1")
             im = Image.new("L", (len(data), 1), None)
             im.putdata(data)
         ImageFile._save(im, op, [("hex", (0,0)+im.size, 0, im.mode)])
diff --git a/Tests/test_file_pdf.py b/Tests/test_file_pdf.py
index 9a4faff3f..e31878aef 100644
--- a/Tests/test_file_pdf.py
+++ b/Tests/test_file_pdf.py
@@ -15,18 +15,12 @@ def helper_save_as_pdf(mode):
     assert_greater(os.path.getsize(outfile), 0)
 
 
-# FIXME: 1-mode test "fails" because it produces a warning.
-# https://travis-ci.org/hugovk/Pillow/builds/24916085
-# /home/travis/build/hugovk/Pillow/PIL/PdfImagePlugin.py:147:
-# DeprecationWarning: tostring() is deprecated. Please call tobytes() instead.
-#   data = im.tostring("raw", "1")
-#
-# def test_monochrome():
-#     # Arrange
-#     mode = "1"
-#
-#     # Act / Assert
-#     helper_save_as_pdf(mode)
+def test_monochrome():
+    # Arrange
+    mode = "1"
+
+    # Act / Assert
+    helper_save_as_pdf(mode)
 
 
 def test_greyscale():