diff --git a/exec.c b/exec.c
index 5e9a5b7..5945496 100644
--- a/exec.c
+++ b/exec.c
@@ -3363,6 +3363,8 @@ static int subpage_register (subpage_t *mmio, uint32_t start, uint32_t end,
            mmio, start, end, idx, eidx, memory);
 #endif
     memory = (memory >> IO_MEM_SHIFT) & (IO_MEM_NB_ENTRIES - 1);
+    if ((memory & ~TARGET_PAGE_MASK) == IO_MEM_RAM)
+        memory = IO_MEM_UNASSIGNED;
     for (; idx <= eidx; idx++) {
         mmio->sub_io_index[idx] = memory;
         mmio->region_offset[idx] = region_offset;