diff --git a/src/eval-apply.c b/src/eval-apply.c
index 05f2d234..4bb60709 100644
--- a/src/eval-apply.c
+++ b/src/eval-apply.c
@@ -877,7 +877,11 @@ begin_expand:
               else if (TYPE (R1) == TPORT)
                 input = set_current_input_port (R1);
               else
-                assert_msg (0, "0");
+                {
+                  eputs ("begin_expand failed, R1=");
+                  display_error_ (R1);
+                  assert_msg (0, "begin-expand-boom 0");
+                }
 
               push_cc (input, R2, R0, cell_vm_return);
               x = read_input_file_env (R0);