diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index c2f3787..068176e 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -929,7 +929,7 @@
 
 	// Now u < (v<<B), compute lower bits in the same way.
 	// Choose shift = B-1 again.
-	s := B
+	s := B - 1
 	qhat := *temps[depth]
 	qhat.clear()
 	qhat.divRecursiveStep(u[s:].norm(), v[s:], depth+1, tmp, temps)