diff --git a/base64.vp b/base64.vp
index eb39e33..3123266 100644
--- a/base64.vp
+++ b/base64.vp
@@ -56,11 +56,13 @@ def b64decode(s: str, out: uint8*) -> sizet:
         val = 0
         # Parse 6-bit values sequentially
         c = s[i] - 43
-        val |= (b64_dec_tab[c] & 0x3F) << 18
+        # 整数提升取消后，`& 0x3F` 的无损常量适配会把左操作数留成 i8（位宽仅 8），<< 18 / << 12 越界报错；
+        # 显式 uint32() 把左操作数加宽到 32 位后再移位（与上方编码段 uint32(...) << 16 的先例一致）。
+        val |= uint32(b64_dec_tab[c] & 0x3F) << 18
         c = s[i+1] - 43
-        val |= (b64_dec_tab[c] & 0x3F) << 12
+        val |= uint32(b64_dec_tab[c] & 0x3F) << 12
         c = s[i+2] - 43
-        if s[i+2] != '=': val |= (b64_dec_tab[c] & 0x3F) << 6
+        if s[i+2] != '=': val |= uint32(b64_dec_tab[c] & 0x3F) << 6  # 同上：i8 左操作数左移 6 位会截断高位（静默回绕）
         c = s[i+3] - 43
         if s[i+3] != '=': val |= (b64_dec_tab[c] & 0x3F)
         # Write recovered bytes