diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 8685f02e7e..e378033797 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -184,7 +184,11 @@ static int mmap_frag(abi_ulong real_start,
 }
 
 #if HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 64
+#ifdef TARGET_AARCH64
+# define TASK_UNMAPPED_BASE  0x5500000000
+#else
 # define TASK_UNMAPPED_BASE  (1ul << 38)
+#endif
 #else
 # define TASK_UNMAPPED_BASE  0x40000000
 #endif