diff --git a/qemu/scripts/decodetree.py b/qemu/scripts/decodetree.py
index 7913944a..a778cfd7 100644
--- a/qemu/scripts/decodetree.py
+++ b/qemu/scripts/decodetree.py
@@ -918,7 +918,7 @@ class Tree:
 
 def build_tree(pats, outerbits, outermask):
     # Find the intersection of all remaining fixedmask.
-    innermask = ~outermask
+    innermask = ~outermask & insnmask
     for i in pats:
         innermask &= i.fixedmask