diff --git a/cheroot/server.py b/cheroot/server.py
index 0abf24f466..47bca319f8 100644
--- a/cheroot/server.py
+++ b/cheroot/server.py
@@ -217,7 +217,7 @@ def __call__(self, rfile, hdict=None):  # noqa: C901  # FIXME
                 # NOTE: The latter is equivalent and does not.
                 # It's a continuation line.
                 if k is None:
-                    raise ValueError("Illegal continuation line.")
+                    raise ValueError('Illegal continuation line.')
                 v = line.strip()
             else:
                 try: