diff --git a/libmproxy/netstring.py b/libmproxy/netstring.py
index a7b7a5372..669e19e35 100644
--- a/libmproxy/netstring.py
+++ b/libmproxy/netstring.py
@@ -23,8 +23,6 @@ class FileEncoder(object):
             
             data -- A string to be encoded and written
         """  
-        if not isinstance(data, str):
-            raise ValueError("data should be of type 'str'")
         write = self.file_out.write
         write(header(data))
         write(data)
@@ -81,8 +79,6 @@ class Decoder(object):
             
             data -- A string containing complete or partial netstring data
         """
-        if not isinstance(data, str):
-            raise ValueError("data should be of type 'str'")
         self.data_pos = 0 
         self.string_start = 0        
         while self.data_pos < len(data):        
diff --git a/test/test_netstring.py b/test/test_netstring.py
index 482859600..3c56acd9a 100644
--- a/test/test_netstring.py
+++ b/test/test_netstring.py
@@ -46,6 +46,16 @@ class uNetstring(libpry.AutoTree):
                     decoded_data.append(s)
             assert decoded_data == self.test_data.split()
 
+    def test_errors(self):
+        d = netstring.Decoder()
+        libpry.raises("Illegal digit", list, d.feed("1:foo"))
+        d = netstring.Decoder()
+        libpry.raises("Preceding zero", list, d.feed("01:f"))
+        d = netstring.Decoder(5)
+        libpry.raises("Maximum size", list, d.feed("500:f"))
+        d = netstring.Decoder()
+        libpry.raises("Illegal digit", list, d.feed(":f"))
+