diff --git a/src/cmd/compile/internal/gc/fmt.go b/src/cmd/compile/internal/gc/fmt.go
index 41502268a7..c0d82d8c16 100644
--- a/src/cmd/compile/internal/gc/fmt.go
+++ b/src/cmd/compile/internal/gc/fmt.go
@@ -1754,7 +1754,11 @@ func fldconv(f *types.Field, flag FmtFlag, mode fmtMode, depth int) string {
 
 	var typ string
 	if f.Isddd() {
-		typ = "..." + tmodeString(f.Type.Elem(), mode, depth)
+		var et *types.Type
+		if f.Type != nil {
+			et = f.Type.Elem()
+		}
+		typ = "..." + tmodeString(et, mode, depth)
 	} else {
 		typ = tmodeString(f.Type, mode, depth)
 	}
diff --git a/test/fixedbugs/issue20233.go b/test/fixedbugs/issue20233.go
new file mode 100644
index 0000000000..5734cf44ef
--- /dev/null
+++ b/test/fixedbugs/issue20233.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.
+
+// Issue 20233: panic while formatting an error message
+
+package p
+
+var f = func(...A) // ERROR "type func(....*) is not an expression" ERROR "undefined: A"