--- a/tests/test-install.t	Thu Sep 07 22:27:23 2017 +0900
+++ b/tests/test-install.t	Thu Sep 07 22:36:54 2017 +0900
@@ -81,6 +81,14 @@
   checking encoding (invalidenc)...
    unknown encoding: invalidenc
 
+exception message in JSON
+
+  $ HGENCODING=invalidenc HGUSER= hg debuginstall -Tjson | grep error
+    "defaulttemplateerror": null,
+    "encodingerror": "unknown encoding: invalidenc",
+    "extensionserror": null, (no-pure !)
+    "usernameerror": "no username supplied",
+
 path variables are expanded (~ is the same as $TESTTMP)
   $ mkdir tools
   $ touch tools/testeditor.exe