diff --git a/src/cmd/compile/internal/walk/order.go b/src/cmd/compile/internal/walk/order.go
index 179fbdb99e987..de180a4a8d779 100644
--- a/src/cmd/compile/internal/walk/order.go
+++ b/src/cmd/compile/internal/walk/order.go
@@ -643,7 +643,12 @@ func (o *orderState) stmt(n ir.Node) {
 			indexLHS.Index = o.cheapExpr(indexLHS.Index)
 
 			call := n.Y.(*ir.CallExpr)
-			indexRHS := call.Args[0].(*ir.IndexExpr)
+			arg0 := call.Args[0]
+			// ir.SameSafeExpr skips OCONVNOPs, so we must do the same here (#66096).
+			for arg0.Op() == ir.OCONVNOP {
+				arg0 = arg0.(*ir.ConvExpr).X
+			}
+			indexRHS := arg0.(*ir.IndexExpr)
 			indexRHS.X = indexLHS.X
 			indexRHS.Index = indexLHS.Index
 
diff --git a/test/fixedbugs/issue66096.go b/test/fixedbugs/issue66096.go
new file mode 100644
index 0000000000000..f8621a18b4ec0
--- /dev/null
+++ b/test/fixedbugs/issue66096.go
@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Message struct {
+	Header map[string][]string
+}
+
+func f() {
+	m := Message{Header: map[string][]string{}}
+	m.Header[""] = append([]string(m.Header[""]), "")
+	_ = m
+}