diff --git a/zlib/zdef.vp b/zlib/zdef.vp
index 5da7dae..bfbf3d1 100644
--- a/zlib/zdef.vp
+++ b/zlib/zdef.vp
@@ -131,7 +131,8 @@ class zbit_writer:
         self.ensure((nbits + 7) / 8 + 1)
         for i in range(int(nbits)):
             if value & (UINT(1) << i):
-                self.buf[self.byte_pos] |= (UINT(1) << self.bit_pos)
+                # BYTE(...) 显式窄化：|= 的目标 buf 元素是 8 位（BYTE/uchar），右值是 i32 位图掩码
+                self.buf[self.byte_pos] |= BYTE(UINT(1) << self.bit_pos)
             self.bit_pos += 1
             if self.bit_pos == 8:
                 self.bit_pos = 0
@@ -141,7 +142,8 @@ class zbit_writer:
         self.ensure((nbits + 7) / 8 + 1)
         for i in range(int(nbits - 1), -1, -1):
             if code & (UINT(1) << i):
-                self.buf[self.byte_pos] |= (UINT(1) << self.bit_pos)
+                # BYTE(...) 显式窄化：|= 的目标 buf 元素是 8 位（BYTE/uchar），右值是 i32 位图掩码
+                self.buf[self.byte_pos] |= BYTE(UINT(1) << self.bit_pos)
             self.bit_pos += 1
             if self.bit_pos == 8:
                 self.bit_pos = 0