diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
index afbb67555df..90f8195a50c 100644
--- a/compiler/rustc_middle/src/mir/interpret/mod.rs
+++ b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -570,7 +570,7 @@ pub fn write_target_uint(
 
 #[inline]
 pub fn read_target_uint(endianness: Endian, mut source: &[u8]) -> Result<u128, io::Error> {
-    let mut buf = [0; 16];
+    let mut buf = [0u8; std::mem::size_of::<u128>()];
     source.read(&mut buf)?;
     match endianness {
         Endian::Little => Ok(u128::from_le_bytes(buf)),