diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -279,7 +279,8 @@ def _parseconfig(ui, config):
                 raise IndexError
             ui.setconfig(section, name, value)
         except (IndexError, ValueError):
-            raise util.Abort(_('malformed --config option: %s') % cfg)
+            raise util.Abort(_('malformed --config option: %r '
+                               '(use --config section.name=value)') % cfg)
 
 def _earlygetopt(aliases, args):
     """Return list of values for an option (or aliases).
diff --git a/tests/test-globalopts.out b/tests/test-globalopts.out
--- a/tests/test-globalopts.out
+++ b/tests/test-globalopts.out
@@ -107,11 +107,11 @@ b
 
 %% --config
 quuxfoo
-abort: malformed --config option: 
-abort: malformed --config option: a.b
-abort: malformed --config option: a
-abort: malformed --config option: a.=
-abort: malformed --config option: .b=
+abort: malformed --config option: '' (use --config section.name=value)
+abort: malformed --config option: 'a.b' (use --config section.name=value)
+abort: malformed --config option: 'a' (use --config section.name=value)
+abort: malformed --config option: 'a.=' (use --config section.name=value)
+abort: malformed --config option: '.b=' (use --config section.name=value)
 %% --debug
 changeset:   1:b6c483daf2907ce5825c0bb50f5716226281cc1a
 tag:         tip