diff --git a/fs/mmap/fs_rammap.c b/fs/mmap/fs_rammap.c
index 16ea937659..222f9f40c6 100644
--- a/fs/mmap/fs_rammap.c
+++ b/fs/mmap/fs_rammap.c
@@ -62,7 +62,7 @@ static int unmap_rammap(FAR struct task_group_s *group,
    * simulate the unmapping.
    */
 
-  offset = start - entry->vaddr;
+  offset = (uintptr_t)start - (uintptr_t)entry->vaddr;
   if (offset + length < entry->length)
     {
       ferr("ERROR: Cannot umap without unmapping to the end\n");