diff --git a/requests/utils.py b/requests/utils.py
index f7a36ad7..5c47de98 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -868,7 +868,7 @@ def check_header_validity(header):
         if not pat.match(value):
             raise InvalidHeader("Invalid return character or leading space in header: %s" % name)
     except TypeError:
-        raise InvalidHeader("Value for header {%s:%s} must be of type str or "
+        raise InvalidHeader("Value for header {%s: %s} must be of type str or "
                             "bytes, not %s" % (name, value, type(value)))