diff --git a/R/util.r b/R/util.r
index ef75489..dcd6e5e 100644
--- a/R/util.r
+++ b/R/util.r
@@ -33,7 +33,8 @@ fill_template = function(template, values, required=c()) {
 
     upd = keys %in% names(values)
     is_num = sapply(values, is.numeric)
-    values[is_num] = format(values[is_num], scientific=FALSE, trim=TRUE)
+    if (length(is_num) > 0)
+        values[is_num] = format(values[is_num], scientific=FALSE, trim=TRUE)
     vals[upd] = unlist(values)[keys[upd]]
     if (any(is.na(vals)))
         stop("Template values required but not provided: ",