diff --git a/libcxxabi/src/fallback_malloc.cpp b/libcxxabi/src/fallback_malloc.cpp
--- a/libcxxabi/src/fallback_malloc.cpp
+++ b/libcxxabi/src/fallback_malloc.cpp
@@ -60,8 +60,21 @@
 static const size_t HEAP_SIZE = 512;
 char heap[HEAP_SIZE] __attribute__((aligned));
 
-typedef unsigned short heap_offset;
-typedef unsigned short heap_size;
+template<size_t S>
+struct half_size_uint;
+
+template<>
+struct half_size_uint<8> {
+  typedef unsigned int type;
+};
+
+template<>
+struct half_size_uint<4> {
+  typedef unsigned short type;
+};
+
+typedef half_size_uint<sizeof(void*)>::type heap_offset;
+typedef half_size_uint<sizeof(void*)>::type heap_size;
 
 struct heap_node {
   heap_offset next_node; // offset into heap
diff --git a/libcxxabi/test/test_fallback_malloc.pass.cpp b/libcxxabi/test/test_fallback_malloc.pass.cpp
--- a/libcxxabi/test/test_fallback_malloc.pass.cpp
+++ b/libcxxabi/test/test_fallback_malloc.pass.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cassert>
 #include <cstdio>
 #include <deque>
 
@@ -179,6 +180,8 @@
     std::printf("fallback_malloc ( 32 ) --> %lu\n", (unsigned long) (p - heap));
     if ( !is_fallback_ptr ( p ))
         std::printf("### p is not a fallback pointer!!\n");
+    assert(((intptr_t)p % sizeof(void *) == 0)
+           && "p must be aligned as sizeof(void*)");
 
     print_free_list ();
     fallback_free ( p );