diff --git a/R/tbl-sql.R b/R/tbl-sql.R
index d3129c8cb..844eb3ad0 100644
--- a/R/tbl-sql.R
+++ b/R/tbl-sql.R
@@ -19,12 +19,9 @@ tbl_sql <- function(subclass, src, from, ..., vars = NULL) {
     from <- as.sql(from, con = src$con)
   }
 
+  check_character(vars, allow_null = TRUE)
   vars <- vars %||% dbplyr_query_fields(src$con, from_sql)
 
-  tbl_sql_impl(subclass, src, from, vars)
-}
-
-tbl_sql_impl <- function(subclass, src, from, vars) {
   dplyr::make_tbl(
     c(subclass, "sql", "lazy"),
     src = src,