diff --git a/artiq/firmware/liballoc_list/lib.rs b/artiq/firmware/liballoc_list/lib.rs
index 8af134b0e..f4d93310a 100644
--- a/artiq/firmware/liballoc_list/lib.rs
+++ b/artiq/firmware/liballoc_list/lib.rs
@@ -13,6 +13,7 @@ const MAGIC_BUSY: usize = 0xFEEDFEED;
 struct Header {
     magic: usize,
     size:  usize,
+    _pad:  usize,
     next:  *mut Header
 }
 
diff --git a/artiq/firmware/runtime/runtime.ld b/artiq/firmware/runtime/runtime.ld
index b1a7ad7e3..81138fea3 100644
--- a/artiq/firmware/runtime/runtime.ld
+++ b/artiq/firmware/runtime/runtime.ld
@@ -75,7 +75,7 @@ SECTIONS
         _fstack = . - 16;
     } > runtime
 
-    .heap (NOLOAD) :
+    .heap (NOLOAD) : ALIGN(16)
     {
         _fheap = .;
         . = ORIGIN(runtime) + LENGTH(runtime);