diff -r 7fe960f1a101 Lib/http/client.py
--- a/Lib/http/client.py	Fri Jan 08 01:01:56 2016 -0800
+++ b/Lib/http/client.py	Fri Jan 08 08:37:42 2016 -0800
@@ -146,6 +146,21 @@
 _METHODS_EXPECTING_BODY = {'PATCH', 'POST', 'PUT'}
 
 
+def _encode(data, encoding, name='data'):
+    """Call data.encode(encoding) but show a better error message."""
+    try:
+        return data.encode(encoding)
+    except UnicodeEncodeError as err:
+        raise UnicodeEncodeError(
+            err.encoding,
+            err.object,
+            err.start,
+            err.end,
+            "%s (%.20r) is not valid Latin-1. Use %s.encode('utf-8') "
+            "if you want to send it encoded in UTF-8." %
+            (name.title(), data[err.start:err.end], name)) from None
+
+
 class HTTPMessage(email.message.Message):
     # XXX The only usage of this method is in
     # http.server.CGIHTTPRequestHandler.  Maybe move the code there so
@@ -796,7 +811,7 @@
         self.send(connect_bytes)
         for header, value in self._tunnel_headers.items():
             header_str = "%s: %s\r\n" % (header, value)
-            header_bytes = header_str.encode("latin-1")
+            header_bytes = _encode(header_str, "latin-1", 'header')
             self.send(header_bytes)
         self.send(b'\r\n')
 
@@ -878,7 +893,7 @@
                 if not datablock:
                     break
                 if encode:
-                    datablock = datablock.encode("iso-8859-1")
+                    datablock = _encode(datablock, "iso-8859-1")
                 self.sock.sendall(datablock)
             return
         try:
@@ -1044,7 +1059,7 @@
             raise CannotSendHeader()
 
         if hasattr(header, 'encode'):
-            header = header.encode('ascii')
+            header = _encode(header, 'ascii', 'header')
 
         if not _is_legal_header_name(header):
             raise ValueError('Invalid header name %r' % (header,))
@@ -1052,7 +1067,7 @@
         values = list(values)
         for i, one_value in enumerate(values):
             if hasattr(one_value, 'encode'):
-                values[i] = one_value.encode('latin-1')
+                values[i] = _encode(one_value, 'latin-1', 'value')
             elif isinstance(one_value, int):
                 values[i] = str(one_value).encode('ascii')
 
@@ -1124,7 +1139,7 @@
         if isinstance(body, str):
             # RFC 2616 Section 3.7.1 says that text default has a
             # default charset of iso-8859-1.
-            body = body.encode('iso-8859-1')
+            body = _encode(body, 'iso-8859-1', 'body')
         self.endheaders(body)
 
     def getresponse(self):