diff --git a/src/calibre/srv/http_request.py b/src/calibre/srv/http_request.py
index 98df2e8b19..8b18489f36 100644
--- a/src/calibre/srv/http_request.py
+++ b/src/calibre/srv/http_request.py
@@ -125,13 +125,16 @@ class HTTPHeaderParser(object):
                     val = existing + ', ' + val
             self.hdict[key] = val
 
+        if self.finished:
+            raise ValueError('Header block already terminated')
+
         if line == b'\r\n':
             # Normal end of headers
             commit()
             self.finished = True
             return
 
-        if line[0] in b' \t':
+        if line and line[0] in b' \t':
             # It's a continuation line.
             if not self.lines:
                 raise ValueError('Orphaned continuation line')
diff --git a/src/calibre/srv/tests/http.py b/src/calibre/srv/tests/http.py
index 3a5a55563a..88995a76d2 100644
--- a/src/calibre/srv/tests/http.py
+++ b/src/calibre/srv/tests/http.py
@@ -42,16 +42,16 @@ class TestHTTP(BaseTest):
              'accept-Encoding: two',
              '\r\n', accept_encoding='one, two')
 
-        def parse(line):
-            HTTPHeaderParser()(line)
+        def parse(*lines):
+            lines = list(lines)
+            lines.append(b'\r\n')
+            self.assertRaises(ValueError, HTTPHeaderParser().push, *lines)
 
-        with self.assertRaises(ValueError):
-            parse('Connection:mūs\r\n')
-            parse('Connection\r\n')
-            parse('Connection:a\r\n')
-            parse('Connection:a\n')
-            parse(' Connection:a\n')
-            parse(':a\n')
+        parse(b'Connection:mūs\r\n')
+        parse(b'Connection\r\n')
+        parse(b'Connection:a\r\n', b'\r\n')
+        parse(b' Connection:a\n')
+        parse(b':a\n')
     # }}}
 
     def test_accept_encoding(self):  # {{{