diff --git a/local/validate-patches.R b/local/validate-patches.R
index dbf7f39..19effe4 100644
--- a/local/validate-patches.R
+++ b/local/validate-patches.R
@@ -34,9 +34,12 @@ for (i in seq_along(reg)) {
 # Ambiguous overlap: two entries for the same package with identical platforms
 # and versions.
 keys <- vapply(reg, function(e) {
-  sprintf("%s|%s|%s", e$package,
-          paste(sort(as.character(unlist(e$platforms))), collapse = ","),
-          e$versions)
+  sprintf(
+    "%s|%s|%s",
+    e$package %||% "?",
+    paste(sort(as.character(unlist(e$platforms))), collapse = ","),
+    e$versions %||% "?"
+  )
 }, character(1L))
 dups <- keys[duplicated(keys)]
 if (length(dups) > 0L) {