diff --git a/R/refmodel.R b/R/refmodel.R
index 62a3447f3..3493eea08 100644
--- a/R/refmodel.R
+++ b/R/refmodel.R
@@ -129,6 +129,10 @@
 #' intercept in the reference model is zero, this does not need to hold for the
 #' submodels.
 #'
+#' In multilevel (group-level) terms, function calls on the right-hand side of
+#' the `|` character (e.g., `(1 | gr(group_variable))`, which is possible in
+#' \pkg{brms}) are currently not allowed in \pkg{projpred}.
+#'
 #' For additive models (still an experimental feature), only [mgcv::s()] and
 #' [mgcv::t2()] are currently supported as smooth terms. Furthermore, these need
 #' to be called without any arguments apart from the predictor names (symbols).
diff --git a/man/refmodel-init-get.Rd b/man/refmodel-init-get.Rd
index f76f476fc..d5055ffea 100644
--- a/man/refmodel-init-get.Rd
+++ b/man/refmodel-init-get.Rd
@@ -182,6 +182,10 @@ submodels which \emph{include} an intercept. The reason is that even if the true
 intercept in the reference model is zero, this does not need to hold for the
 submodels.
 
+In multilevel (group-level) terms, function calls on the right-hand side of
+the \code{|} character (e.g., \code{(1 | gr(group_variable))}, which is possible in
+\pkg{brms}) are currently not allowed in \pkg{projpred}.
+
 For additive models (still an experimental feature), only \code{\link[mgcv:s]{mgcv::s()}} and
 \code{\link[mgcv:t2]{mgcv::t2()}} are currently supported as smooth terms. Furthermore, these need
 to be called without any arguments apart from the predictor names (symbols).