diff --git a/R/utils.R b/R/utils.R
index 338b91d..b3b2911 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -1,10 +1,13 @@
 with_warnings <- function(expr) {
   my_warnings <- NULL
+
   w_handler <- function(w) {
     my_warnings <<- c(my_warnings, list(w))
     invokeRestart("muffleWarning")
   }
+
   val <- withCallingHandlers(expr, warning = w_handler)
-  list(output = val, warnings = lapply(my_warnings,
-                                       function(w) jsonlite::unbox(w$message)))
+  list(output = val,
+       warnings = lapply(my_warnings,
+                         function(w) jsonlite::unbox(conditionMessage(w))))
 }