diff --git a/R/collect_n_subject.R b/R/collect_n_subject.R
index 6e94893..c109f17 100644
--- a/R/collect_n_subject.R
+++ b/R/collect_n_subject.R
@@ -272,7 +272,10 @@ collect_n_subject <- function(meta,
 
   # standardize categorical variables
   if (any(c("factor", "character") %in% class_var)) {
-    var <- factor(var, exclude = NULL)
+
+    if (any(c("character") %in% class_var)) {
+      var <- factor(var, exclude = NULL)
+    }
 
     if (all(is.na(var))) {
       levels(var) <- c(levels(var), title["missing"])