diff --git a/gdbstub_arch/src/aarch64/reg/id.rs b/gdbstub_arch/src/aarch64/reg/id.rs
index 9ed5320..27c4ca9 100644
--- a/gdbstub_arch/src/aarch64/reg/id.rs
+++ b/gdbstub_arch/src/aarch64/reg/id.rs
@@ -51,7 +51,7 @@ impl AArch64RegId {
             Self::X(_n @ 0..=30) => Some(core::mem::size_of::<u64>()),
             Self::V(_n @ 0..=31) => Some(core::mem::size_of::<u128>()),
             Self::Pc | Self::Sp | Self::System(_) => Some(core::mem::size_of::<u64>()),
-            _ => None,
+            Self::X(_) | Self::V(_) => unreachable!("invalid core register offset"),
         }
     }