diff --git a/R/output.R b/R/output.R
index 3ddbfa30ae..f65231ffd1 100644
--- a/R/output.R
+++ b/R/output.R
@@ -563,7 +563,9 @@ knit_cnd_format = function(cnd) {
 
 #' @export
 knit_cnd_format.message = function(cnd) {
-  conditionMessage(cnd)
+  # Character vectors are created by `merge_class()` to support
+  # https://github.com/yihui/knitr-examples/blob/master/117-messages.Rmd
+  paste(conditionMessage(cnd), collapse = '')
 }
 
 #' @export
diff --git a/tests/testit/test-output.R b/tests/testit/test-output.R
index 0de1122c3b..9f5f8e492f 100644
--- a/tests/testit/test-output.R
+++ b/tests/testit/test-output.R
@@ -192,3 +192,12 @@ local({
     )
   })
 })
+
+# Character vectors are created by `merge_class()` to support
+# https://github.com/yihui/knitr-examples/blob/master/117-messages.Rmd
+assert('knit_cnd_format.message() supports character vectors',
+  identical(
+    knit_cnd_format(simpleMessage(c("foo", "bar"))),
+    "foobar"
+  )
+)