diff --git a/include/unicorn/unicorn.h b/include/unicorn/unicorn.h
index 68b20e67..474c6afe 100644
--- a/include/unicorn/unicorn.h
+++ b/include/unicorn/unicorn.h
@@ -392,6 +392,7 @@ uc_err uc_hook_del(uch handle, uch *h2);
 
  @handle: handle returned by uc_open()
  @address: starting address of the new memory region to be mapped in.
+    This address will be round down to 8KB boundary
  @size: size of the new memory region to be mapped in. This will be round up to
     the next 8KB boundary.
 
diff --git a/uc.c b/uc.c
index 9cfbeb32..c9043750 100644
--- a/uc.c
+++ b/uc.c
@@ -553,7 +553,7 @@ uc_err uc_mem_map(uch handle, uint64_t address, size_t size)
         return UC_ERR_UCH;
 
     // align to 8KB boundary
-    map_begin[map_count] = address & (~ (8*1024));
+    map_begin[map_count] = address & (~ (8*1024 - 1));
     s = (size + 8*1024 - 1) & (~ (8*1024));
     map_end[map_count] = s + map_begin[map_count];
     uc->memory_map(uc, map_begin[map_count], s);