diff --git a/R/draws-index.R b/R/draws-index.R
index beb83fa..722f414 100644
--- a/R/draws-index.R
+++ b/R/draws-index.R
@@ -68,6 +68,10 @@ iteration_ids.draws_df <- function(x) {
 
 #' @export
 iteration_ids.draws_list <- function(x) {
+  if (!length(x) || !length(x[[1]])) {
+    # no chains or no variables within chains
+    return(integer(0))
+  }
   seq_along(x[[1]][[1]])
 }
 
diff --git a/man/u_scale.Rd b/man/u_scale.Rd
index 87d4edf..6424636 100644
--- a/man/u_scale.Rd
+++ b/man/u_scale.Rd
@@ -19,7 +19,9 @@ and dimension as the input.
 \description{
 Compute rank uniformization for a numeric array. First replace each value by
 its rank. Average rank for ties are used to conserve the number of unique
-values of discrete quantities. Second, uniformize ranks to the scale
-\verb{[1/(2S), 1-1/(2S)]}, where \code{S} is the number of values.
+values of discrete quantities. Second, uniformize ranks using formula
+\code{(r - c) / (S - 2 * c + 1)}, where \code{r} is a rank, \code{S} is the number of
+values, and \code{c} is a fractional offset which defaults to c = 3/8 as
+recommend by Blom (1958).
 }
 \keyword{internal}