diff --git a/src/psaux/psstack.c b/src/psaux/psstack.c
index 797486588..f8d1e8d44 100644
--- a/src/psaux/psstack.c
+++ b/src/psaux/psstack.c
@@ -211,7 +211,7 @@
                      CF2_UInt   idx,
                      CF2_Fixed  val )
   {
-    if ( idx > cf2_stack_count( stack ) )
+    if ( idx >= cf2_stack_count( stack ) )
     {
       CF2_SET_ERROR( stack->error, Stack_Overflow );
       return;