diff --git a/fitparse/base.py b/fitparse/base.py
index b0ebf87..5d3c6a2 100644
--- a/fitparse/base.py
+++ b/fitparse/base.py
@@ -296,7 +296,8 @@ def _parse_definition_message(self, header):
             if (field_size % base_type.size) != 0:
                 warnings.warn(
                     "Invalid field size %d for field '%s' of type '%s' (expected a multiple of %d); falling back to byte encoding." % (
-                    field_size, field.name, base_type.name, base_type.size)
+                    field_size, field.name if field else 'unknown_%d' % field_def_num,
+                    base_type.name, base_type.size)
                 )
                 base_type = BASE_TYPE_BYTE
 
diff --git a/tests/test.py b/tests/test.py
index e4851d5..1677b88 100755
--- a/tests/test.py
+++ b/tests/test.py
@@ -2,6 +2,7 @@
 
 import csv
 import datetime
+import io
 import os
 from struct import pack
 import warnings
@@ -74,6 +75,13 @@ def testfile(filename):
 
 
 class FitFileTestCase(unittest.TestCase):
+    def test_unknown_field_with_incompatible_size_warns(self):
+        definition = b'\x40\x00\x00' + pack('<HB', 65535, 1) + bytes([0, 1, 0x84])
+        header = pack('<2BHI4s', 14, 16, 152, len(definition), b'.FIT') + b'\x00\x00'
+
+        with self.assertWarnsRegex(UserWarning, 'unknown_0'):
+            FitFile(io.BytesIO(header + definition), check_crc=False).parse()
+
 
     def test_basic_file_with_one_record(self, endian='<'):
         f = FitFile(generate_fitfile(endian=endian))