diff --git a/artiq/gateware/amp/mailbox.py b/artiq/gateware/amp/mailbox.py
index 4c93557df..7c60056d1 100644
--- a/artiq/gateware/amp/mailbox.py
+++ b/artiq/gateware/amp/mailbox.py
@@ -12,10 +12,10 @@ class Mailbox(Module):
         values = Array([Signal(32) for _ in range(size)])
         for i in self.i1, self.i2:
             self.sync += [
-                i.dat_r.eq(values[i.adr & 0xff]),
+                i.dat_r.eq(values[i.adr[:bits_for(size-1)]]),
                 i.ack.eq(0),
                 If(i.cyc & i.stb & ~i.ack,
                     i.ack.eq(1),
-                    If(i.we, values[i.adr & 0xff].eq(i.dat_w))
+                    If(i.we, values[i.adr[:bits_for(size-1)]].eq(i.dat_w))
                 )
             ]