diff --git a/tgt-vvp/eval_expr.c b/tgt-vvp/eval_expr.c
index 3bcbf2717..d4f341cd7 100644
--- a/tgt-vvp/eval_expr.c
+++ b/tgt-vvp/eval_expr.c
@@ -92,6 +92,7 @@ unsigned long get_number_immediate(ivl_expr_t ex)
 		    case '0':
 		      break;
 		    case '1':
+		      assert(idx < 8*sizeof(imm));
 		      imm |= 1UL << idx;
 		      break;
 		    default: