diff --git a/tools/rosbag/src/rosbag/bag.py b/tools/rosbag/src/rosbag/bag.py
index 1f9cf36f3e..3130b43b73 100644
--- a/tools/rosbag/src/rosbag/bag.py
+++ b/tools/rosbag/src/rosbag/bag.py
@@ -1586,6 +1586,7 @@ def _read_uint32(f): return _unpack_uint32(f.read(4))
 def _read_uint64(f): return _unpack_uint64(f.read(8))
 def _read_time  (f): return _unpack_time  (f.read(8))
 
+def _decode_str(v):    return v if type(v) is str else v.decode()
 def _unpack_uint8(v):  return struct.unpack('<B', v)[0]
 def _unpack_uint32(v): return struct.unpack('<L', v)[0]
 def _unpack_uint64(v): return struct.unpack('<Q', v)[0]
@@ -1634,8 +1635,7 @@ def _read_field(header, field, unpack_fn):
     
     return value
 
-def _read_str_field   (header, field):
-    return _read_field(header, field, lambda v: v)
+def _read_str_field   (header, field): return _read_field(header, field, _decode_str)
 def _read_uint8_field (header, field): return _read_field(header, field, _unpack_uint8)
 def _read_uint32_field(header, field): return _read_field(header, field, _unpack_uint32)
 def _read_uint64_field(header, field): return _read_field(header, field, _unpack_uint64)