diff --git a/src/cmd/compile/internal/gc/swt.go b/src/cmd/compile/internal/gc/swt.go
index 469af86aa6..dc285ae91c 100644
--- a/src/cmd/compile/internal/gc/swt.go
+++ b/src/cmd/compile/internal/gc/swt.go
@@ -610,6 +610,11 @@ func checkDupExprCases(exprname *Node, clauses []*Node) {
 				if ct := consttype(n); ct < 0 || ct == CTBOOL {
 					continue
 				}
+				// If the value has no type, we have
+				// already printed an error about it.
+				if n.Type == nil {
+					continue
+				}
 
 				val := n.Val().Interface()
 				prev, dup := seen[val]
diff --git a/test/fixedbugs/issue21988.go b/test/fixedbugs/issue21988.go
new file mode 100644
index 0000000000..850e0398d6
--- /dev/null
+++ b/test/fixedbugs/issue21988.go
@@ -0,0 +1,17 @@
+// 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.
+
+// Issue 21988: panic on switch case with invalid value
+
+package p
+
+const X = Wrong(0) // ERROR "undefined: Wrong"
+
+func _() {
+	switch 0 {
+	case X:
+	}
+}