diff --git a/eth2/utils/ssz/src/encode/impls.rs b/eth2/utils/ssz/src/encode/impls.rs
index 04492a1f2..e0e2d9dbc 100644
--- a/eth2/utils/ssz/src/encode/impls.rs
+++ b/eth2/utils/ssz/src/encode/impls.rs
@@ -24,6 +24,11 @@ impl_encodable_for_uint!(u8, 8);
 impl_encodable_for_uint!(u16, 16);
 impl_encodable_for_uint!(u32, 32);
 impl_encodable_for_uint!(u64, 64);
+
+#[cfg(target_pointer_width = "32")]
+impl_encodable_for_uint!(usize, 32);
+
+#[cfg(target_pointer_width = "64")]
 impl_encodable_for_uint!(usize, 64);
 
 /// The SSZ "union" type.