diff --git a/distributed/protocol/numpy.py b/distributed/protocol/numpy.py
index 5ed22fb554d..e146df937e8 100644
--- a/distributed/protocol/numpy.py
+++ b/distributed/protocol/numpy.py
@@ -50,7 +50,7 @@ def serialize_numpy_ndarray(x):
 
     data = x.view('u1').data
 
-    if blosc and len(data) > 1e5:
+    if blosc and data.nbytes > 1e5:
         frames = frame_split_size([data])
         if sys.version_info.major == 2:
             frames = [ensure_bytes(frame) for frame in frames]
@@ -58,7 +58,7 @@ def serialize_numpy_ndarray(x):
         out = []
         compression = []
         for frame in frames:
-            sample = byte_sample(frame, 10000 * size, 5)
+            sample = byte_sample(frame, 10000 // size * size, 5)
             csample = blosc.compress(sample, typesize=size, cname='lz4', clevel=3)
             if len(csample) < 0.8 * len(sample):
                 compressed = blosc.compress(frame, typesize=size, cname='lz4', clevel=5)
diff --git a/distributed/protocol/tests/test_numpy.py b/distributed/protocol/tests/test_numpy.py
index ebecd8ee6e1..8b976dedbb2 100644
--- a/distributed/protocol/tests/test_numpy.py
+++ b/distributed/protocol/tests/test_numpy.py
@@ -47,6 +47,8 @@ def test_serialize():
          np.ones(shape=(5,), dtype=('f8', 32)),
          np.ones(shape=(5,), dtype=[('x', 'f8', 32)]),
          np.array([(1, 'abc')], dtype=[('x', 'i4'), ('s', object)]),
+         np.zeros(5000, dtype=[('x%d'%i,'<f8') for i in range(4)]),
+         np.zeros(5000, dtype='S32'),
          np.ones(shape=(5, 6)).astype(dtype=[('total', '<f8'), ('n', '<f8')])])
 def test_dumps_serialize_numpy(x):
     header, frames = serialize(x)