diff --git a/R/utils.R b/R/utils.R
index b3b2911..cd24f4b 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -2,12 +2,11 @@ with_warnings <- function(expr) {
   my_warnings <- NULL
 
   w_handler <- function(w) {
-    my_warnings <<- c(my_warnings, list(w))
+    my_warnings <<- c(my_warnings, jsonlite::unbox(conditionMessage(w)))
     invokeRestart("muffleWarning")
   }
 
   val <- withCallingHandlers(expr, warning = w_handler)
   list(output = val,
-       warnings = lapply(my_warnings,
-                         function(w) jsonlite::unbox(conditionMessage(w))))
+       warnings = my_warnings)
 }