diff --git a/netlib/strutils.py b/netlib/strutils.py
index cda706517..7d49e56e5 100644
--- a/netlib/strutils.py
+++ b/netlib/strutils.py
@@ -119,7 +119,7 @@ def is_mostly_bin(s):
     return sum(
         i < 9 or 13 < i < 32 or 126 < i
         for i in six.iterbytes(s[:100])
-    ) > 30
+    ) / len(s[:100]) > 0.3
 
 
 def is_xml(s):
diff --git a/test/netlib/test_strutils.py b/test/netlib/test_strutils.py
index f88e33ed6..68bfdb942 100644
--- a/test/netlib/test_strutils.py
+++ b/test/netlib/test_strutils.py
@@ -3,6 +3,13 @@ import six
 from netlib import strutils, tutils
 
 
+def test_always_bytes():
+    assert strutils.always_bytes(bytes(bytearray(range(256)))) == bytes(bytearray(range(256)))
+    assert strutils.always_bytes("foo") == b"foo"
+    with tutils.raises(ValueError):
+        strutils.always_bytes(u"\u2605", "ascii")
+
+
 def test_native():
     with tutils.raises(TypeError):
         strutils.native(42)
@@ -31,6 +38,9 @@ def test_escape_control_characters():
         u'=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~.'
     )
 
+    with tutils.raises(ValueError):
+        strutils.escape_control_characters(b"foo")
+
 
 def test_bytes_to_escaped_str():
     assert strutils.bytes_to_escaped_str(b"foo") == "foo"
@@ -68,6 +78,11 @@ def test_escaped_str_to_bytes():
             strutils.escaped_str_to_bytes(b"very byte")
 
 
+def test_is_mostly_bin():
+    assert not strutils.is_mostly_bin(b"foo\xFF")
+    assert strutils.is_mostly_bin(b"foo" + b"\xFF" * 10)
+
+
 def test_is_xml():
     assert not strutils.is_xml(b"foo")
     assert strutils.is_xml(b"<foo")