diff --git a/R/variables.R b/R/variables.R
index 63d62cd..e0c732a 100644
--- a/R/variables.R
+++ b/R/variables.R
@@ -235,6 +235,9 @@ check_existing_variables <- function(variables, x, regex = FALSE,
     return(NULL)
   }
 
+  # do not allow NA values in variables
+  checkmate::assertCharacter(variables, any.missing = FALSE)
+
   regex <- as_one_logical(regex)
   scalar <- as_one_logical(scalar)
   exclude <- as_one_logical(exclude)