diff --git a/R/blockApply.R b/R/blockApply.R
index c062d65..d55f9d8 100644
--- a/R/blockApply.R
+++ b/R/blockApply.R
@@ -92,7 +92,7 @@ getAutoBPPARAM <- function() S4Arrays:::get_user_option("auto.BPPARAM")
 
 set_grid_context <- function(effective_grid,
                              current_block_id,
-                             current_viewport,
+                             current_viewport=NULL,
                              envir=parent.frame(1))
 {
     if (!is.null(effective_grid))
diff --git a/man/blockApply.Rd b/man/blockApply.Rd
index 303116f..36a6475 100644
--- a/man/blockApply.Rd
+++ b/man/blockApply.Rd
@@ -48,7 +48,7 @@ getAutoBPPARAM()
 setAutoBPPARAM(BPPARAM=NULL)
 
 ## For testing/debugging callback functions:
-set_grid_context(effective_grid, current_block_id, current_viewport,
+set_grid_context(effective_grid, current_block_id, current_viewport=NULL,
                  envir=parent.frame(1))
 }