src/go/types/decl.go                           |  1 +
 src/go/types/testdata/fixedbugs/issue48819.src | 15 +++++++++++++++
 src/go/types/type.go                           |  3 +++

diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 9211febc6da1620a43e9867af2f37c149a6f0753..ec173ad1ccc9cb62ed3ae292ae303e5e1f23c057 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -343,6 +343,7 @@ 				}
 				if tn == t.obj {
 					check.cycleError(path[i:])
 					t.info = invalid
+					t.underlying = Typ[Invalid]
 					return t.info
 				}
 			}
diff --git a/src/go/types/testdata/fixedbugs/issue48819.src b/src/go/types/testdata/fixedbugs/issue48819.src
new file mode 100644
index 0000000000000000000000000000000000000000..9262110ea09e71e96c3494186beaba6c6c5f4db5
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue48819.src
@@ -0,0 +1,15 @@
+// Copyright 2021 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
+
+import "unsafe"
+
+type T /* ERROR illegal cycle in declaration of T */ struct {
+	T
+}
+
+func _(t T) {
+	_ = unsafe.Sizeof(t) // should not go into infinite recursion here
+}
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 2660ce4408c1b640f67bf517ca72d3819055907e..20c4bec0bce241ac84fae75a309f90e81948f1e6 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -759,6 +759,9 @@ }
 
 func (t *_TypeParam) Bound() *Interface {
 	iface := asInterface(t.bound)
+	if iface == nil {
+		return &emptyInterface
+	}
 	// use the type bound position if we have one
 	pos := token.NoPos
 	if n, _ := t.bound.(*Named); n != nil {