diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2423,7 +2423,8 @@ def debugextensions(ui, **opts):
                  _('  location: %s\n'), extsource or "")
 
         fm.condwrite(ui.verbose and exttestedwith, 'testedwith',
-                 _('  tested with: %s\n'), ' '.join(exttestedwith))
+                     _('  tested with: %s\n'),
+                     fm.formatlist(exttestedwith, name='ver'))
 
         fm.condwrite(ui.verbose and extbuglink, 'buglink',
                  _('  bug reporting: %s\n'), extbuglink or "")
diff --git a/tests/test-debugextensions.t b/tests/test-debugextensions.t
--- a/tests/test-debugextensions.t
+++ b/tests/test-debugextensions.t
@@ -48,36 +48,39 @@
     "buglink": "",
     "name": "color",
     "source": "*/hgext/color.py*", (glob)
-    "testedwith": "internal"
+    "testedwith": ["internal"]
    },
    {
     "buglink": "",
     "name": "ext1",
     "source": "*/extwithoutinfos.py*", (glob)
-    "testedwith": ""
+    "testedwith": []
    },
    {
     "buglink": "",
     "name": "histedit",
     "source": "*/hgext/histedit.py*", (glob)
-    "testedwith": "internal"
+    "testedwith": ["internal"]
    },
    {
     "buglink": "",
     "name": "mq",
     "source": "*/hgext/mq.py*", (glob)
-    "testedwith": "internal"
+    "testedwith": ["internal"]
    },
    {
     "buglink": "",
     "name": "patchbomb",
     "source": "*/hgext/patchbomb.py*", (glob)
-    "testedwith": "internal"
+    "testedwith": ["internal"]
    },
    {
     "buglink": "",
     "name": "rebase",
     "source": "*/hgext/rebase.py*", (glob)
-    "testedwith": "internal"
+    "testedwith": ["internal"]
    }
   ]
+
+  $ hg debugextensions -T '{ifcontains("internal", testedwith, "", "{name}\n")}'
+  ext1