diff --git a/Tests/test_file_libtiff.py b/Tests/test_file_libtiff.py
index 2ddb2263a..6834b7256 100644
--- a/Tests/test_file_libtiff.py
+++ b/Tests/test_file_libtiff.py
@@ -235,7 +235,10 @@ class TestFileLibTiff(LibTiffTestCase):
     def test_custom_metadata(self):
         custom = {
             37000: 4,
-            37001: 4.2
+            37001: 4.2,
+            37002: 'custom tag value',
+            37003: u'custom tag value',
+            37004: b'custom tag value'
         }
 
         libtiff_version = TiffImagePlugin._libtiff_version()
@@ -256,6 +259,8 @@ class TestFileLibTiff(LibTiffTestCase):
 
             reloaded = Image.open(out)
             for tag, value in custom.items():
+                if libtiff and isinstance(value, bytes):
+                    value = value.decode()
                 self.assertEqual(reloaded.tag_v2[tag], value)
 
     def test_int_dpi(self):
diff --git a/src/PIL/TiffImagePlugin.py b/src/PIL/TiffImagePlugin.py
index a3db8e15c..be24624d6 100644
--- a/src/PIL/TiffImagePlugin.py
+++ b/src/PIL/TiffImagePlugin.py
@@ -1519,12 +1519,16 @@ def _save(im, fp, filename):
                                           getattr(im, 'tag_v2', {}).items(),
                                           legacy_ifd.items()):
             # Libtiff can only process certain core items without adding
-            # them to the custom dictionary. Support has only been been added
-            # for int and float values
+            # them to the custom dictionary.
+            # Support for custom items has only been been added
+            # for int, float, unicode, string and byte values
             if tag not in TiffTags.LIBTIFF_CORE:
+                if TiffTags.lookup(tag).type == TiffTags.UNDEFINED:
+                    continue
                 if (distutils.version.StrictVersion(_libtiff_version()) <
                     distutils.version.StrictVersion("4.0")) \
-                   or not (isinstance(value, int) or isinstance(value, float)):
+                   or not (isinstance(value, (int, float, str, bytes)) or
+                           (not py3 and isinstance(value, unicode))):  # noqa: F821
                     continue
             if tag not in atts and tag not in blocklist:
                 if isinstance(value, str if py3 else unicode):  # noqa: F821
diff --git a/src/encode.c b/src/encode.c
index 3c2f90f18..13da1b999 100644
--- a/src/encode.c
+++ b/src/encode.c
@@ -885,9 +885,11 @@ PyImaging_LibTiffEncoderNew(PyObject* self, PyObject* args)
             }
         } else if (PyBytes_Check(value)) {
             TRACE(("Setting from Bytes: %d, %s \n", key_int, PyBytes_AsString(value)));
-            status = ImagingLibTiffSetField(&encoder->state,
-                                            (ttag_t) PyInt_AsLong(key),
-                                            PyBytes_AsString(value));
+            if (is_core_tag || !ImagingLibTiffMergeFieldInfo(&encoder->state, TIFF_ASCII, key_int)) {
+                status = ImagingLibTiffSetField(&encoder->state,
+                                                (ttag_t) PyInt_AsLong(key),
+                                                PyBytes_AsString(value));
+            }
         } else if (PyTuple_Check(value)) {
             Py_ssize_t len,i;
             float *floatav;