diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index bff278b7ae..795bdcdd35 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/src/cmd/compile/internal/gc/typecheck.go
@@ -3288,10 +3288,10 @@ func checkassign(stmt *Node, n *Node) {
 
 	if n.Op == ODOT && n.Left.Op == OINDEXMAP {
 		yyerror("cannot assign to struct field %v in map", n)
-		return
+	} else {
+		yyerror("cannot assign to %v", n)
 	}
-
-	yyerror("cannot assign to %v", n)
+	n.Type = nil
 }
 
 func checkassignlist(stmt *Node, l Nodes) {
diff --git a/test/fixedbugs/issue20813.go b/test/fixedbugs/issue20813.go
new file mode 100644
index 0000000000..b931aea592
--- /dev/null
+++ b/test/fixedbugs/issue20813.go
@@ -0,0 +1,11 @@
+// errorcheck
+
+// Copyright 2017 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
+
+func f() {
+	1 = 2 // ERROR "cannot assign to 1"
+}