diff --git a/src_base/xeve_bsw.c b/src_base/xeve_bsw.c
index 73e5bc2..6ca12af 100644
--- a/src_base/xeve_bsw.c
+++ b/src_base/xeve_bsw.c
@@ -198,7 +198,7 @@ int xeve_bsw_write(XEVE_BSW * bs, u32 val, int len) /* len(1 ~ 32) */
     int leftbits;
 
     xeve_assert(bs);
-    xeve_assert(len <= 0); // to avoid shifting by a negative value
+    xeve_assert(len > 0); // to avoid shifting by a negative value
 
     leftbits = bs->leftbits;
     val <<= (32 - len);