diff --git a/src/reconstruct.cpp b/src/reconstruct.cpp
index e3e56bbbdf..f9862c6395 100644
--- a/src/reconstruct.cpp
+++ b/src/reconstruct.cpp
@@ -106,7 +106,9 @@ SEXP ffi_dplyr_reconstruct(SEXP data, SEXP template_) {
     Rf_errorcall(R_NilValue, "Internal error: `template` must have a `row.names` attribute.");
   }
 
-  // Make an ALTREP wrapper if possible, since the underlying data doesn't change
+  // Make an ALTREP wrapper if possible, since the underlying data doesn't change.
+  // Won't actually make an ALTREP wrapper unless there are >64 columns
+  // (internally controlled by R).
 #if R_VERSION >= R_Version(3, 6, 0)
   data = PROTECT(R_shallow_duplicate_attr(data));
 #else