diff -r d7c8914a9c4d Lib/wsgiref/validate.py
--- a/Lib/wsgiref/validate.py	Sat Apr 21 18:32:56 2012 +0100
+++ b/Lib/wsgiref/validate.py	Mon Apr 23 13:16:51 2012 -0400
@@ -77,7 +77,7 @@
 
 * That wsgi.input is used properly:
 
-  - .read() is called with zero or one argument
+  - .read() is called with one argument
 
   - That it returns a string
 
@@ -194,23 +194,24 @@
         self.input = wsgi_input
 
     def read(self, *args):
-        assert_(len(args) == 1)
+        assert_(len(args) == 1, "read() takes one argument (%d given)" % (len(args),))
         v = self.input.read(*args)
-        assert_(type(v) is bytes)
+        assert_(type(v) is bytes, "read() must return bytes, got %r" % (v,))
         return v
 
     def readline(self, *args):
-        assert_(len(args) <= 1)
+        assert_(len(args) <= 1, "readline() takes zero or one arguments (%d given)" % (len(args),))
         v = self.input.readline(*args)
-        assert_(type(v) is bytes)
+        assert_(type(v) is bytes, "readline() must return bytes, got %r" % (v,))
         return v
 
     def readlines(self, *args):
-        assert_(len(args) <= 1)
+        assert_(len(args) <= 1, "readlines() takes zero or one arguments (%d given)" % (len(args),))
         lines = self.input.readlines(*args)
-        assert_(type(lines) is list)
+        assert_(type(lines) is list, "readlines() must return a list, got %r" % (lines,))
         for line in lines:
-            assert_(type(line) is bytes)
+            assert_(type(line) is bytes,
+                "Each line returned by readlines() must be of type bytes, got %r" % (line,))
         return lines
 
     def __iter__(self):
@@ -229,7 +230,7 @@
         self.errors = wsgi_errors
 
     def write(self, s):
-        assert_(type(s) is str)
+        check_string_type(s, "Argument to write()")
         self.errors.write(s)
 
     def flush(self):
@@ -248,7 +249,7 @@
         self.writer = wsgi_writer
 
     def __call__(self, s):
-        assert_(type(s) is bytes)
+        assert_(type(s) is bytes, "write() must be called with bytes, got %r" % (s,))
         self.writer(s)
 
 class PartialIteratorWrapper:
@@ -395,7 +396,8 @@
         assert_(type(item) is tuple,
             "Individual headers (%r) must be of type tuple: %r"
             % (item, type(item)))
-        assert_(len(item) == 2)
+        assert_(len(item) == 2,
+                "Individual headers must only have two items: %r" % (item,))
         name, value = item
         name = check_string_type(name, "Header name")
         value = check_string_type(value, "Header value")