diff --git a/src/go/types/named.go b/src/go/types/named.go
index 07c837d14a..d29c67d4eb 100644
--- a/src/go/types/named.go
+++ b/src/go/types/named.go
@@ -332,7 +332,7 @@ func (check *Checker) completeMethod(env *Environment, m *Func) {
 // TODO(rfindley): eliminate this function or give it a better name.
 func safeUnderlying(typ Type) Type {
 	if t, _ := typ.(*Named); t != nil {
-		return t.resolve(nil).underlying
+		return t.underlying
 	}
 	return typ.Underlying()
 }
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 999099572c..25629dca8a 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -177,7 +177,9 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 		var newTArgs []Type
-		assert(t.targs.Len() == t.orig.TypeParams().Len())
+		if t.targs.Len() != t.orig.TypeParams().Len() {
+			return Typ[Invalid] // error reported elsewhere
+		}
 
 		// already instantiated
 		dump(">>> %s already instantiated", t)
diff --git a/src/go/types/testdata/fixedbugs/issue46461.go2 b/src/go/types/testdata/fixedbugs/issue46461.go2
new file mode 100644
index 0000000000..bfeaf3a966
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue46461.go2
@@ -0,0 +1,11 @@
+// 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
+
+type T[U interface{ M() T[U] }] int
+
+type X int
+
+func (X) M() T[X] { return 0 }
diff --git a/src/go/types/testdata/fixedbugs/issue48529.go2 b/src/go/types/testdata/fixedbugs/issue48529.go2
new file mode 100644
index 0000000000..4f92dec7fe
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue48529.go2
@@ -0,0 +1,11 @@
+// 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
+
+type T[U interface{ M() T /* ERROR "got 2 arguments but 1 type parameters" */ [U, int] }] int
+
+type X int
+
+func (X) M() T[X] { return 0 }