diff --git a/compiler/optimizer/join.go b/compiler/optimizer/join.go
index 339ae2567..e990267c2 100644
--- a/compiler/optimizer/join.go
+++ b/compiler/optimizer/join.go
@@ -150,6 +150,8 @@ func equiJoinKeyExprs(e dag.Expr, leftAlias, rightAlias string) (left, right dag
 	if lhsFirst != leftAlias || rhsFirst != rightAlias {
 		return nil, nil, false
 	}
+	// This next step will mutate the input expression, do a copy here.
+	lhs, rhs = dag.CopyExpr(lhs), dag.CopyExpr(rhs)
 	stripFirstThisPathComponent(lhs)
 	stripFirstThisPathComponent(rhs)
 	return lhs, rhs, true
diff --git a/runtime/ztests/op/join-expr.yaml b/runtime/ztests/op/join-expr.yaml
index b85df0f35..236aedacd 100644
--- a/runtime/ztests/op/join-expr.yaml
+++ b/runtime/ztests/op/join-expr.yaml
@@ -40,3 +40,17 @@ outputs:
       {a:1::int32,s:"a",b:6::int32}
       {a:2::int32,s:"B",b:6::int32}
       {a:3::int32,s:"c",b:error("missing")}
+
+---
+
+# Issue #6361.
+spq: |
+  values {a:1,x:5},{a:2,x:6}
+  | join (
+    values {b:1,y:7},{b:2,y:4}
+  ) as {l,r} on l.a=r.b and l.x > r.y
+
+vector: true
+
+output: |
+  {l:{a:2,x:6},r:{b:2,y:4}}