diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index fbfe3c81ff..b4035e16b3 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -491,6 +491,13 @@ func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Fun
 // Otherwise it returns (typ, false).
 func deref(typ Type) (Type, bool) {
 	if p, _ := typ.(*Pointer); p != nil {
+		// p.base should never be nil, but be conservative
+		if p.base == nil {
+			if debug {
+				panic("pointer with nil base type (possibly due to an invalid cyclic declaration)")
+			}
+			return Typ[Invalid], true
+		}
 		return p.base, true
 	}
 	return typ, false
diff --git a/src/cmd/compile/internal/types2/testdata/check/cycles.src b/src/cmd/compile/internal/types2/testdata/check/cycles.src
index b2ee8ecd5f..998f9f7da9 100644
--- a/src/cmd/compile/internal/types2/testdata/check/cycles.src
+++ b/src/cmd/compile/internal/types2/testdata/check/cycles.src
@@ -45,6 +45,7 @@ type (
 
 	// pointers
 	P0 *P0
+	PP *struct{ PP.f /* ERROR no field or method f */ }
 
 	// functions
 	F0 func(F0)
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 7fcb196c5a..af195c08a4 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -21,13 +21,10 @@ type Type interface {
 // under must only be called when a type is known
 // to be fully set up.
 func under(t Type) Type {
-	switch t := t.(type) {
-	case *Named:
+	if t, _ := t.(*Named); t != nil {
 		return t.under()
-	case *TypeParam:
-		return t.iface()
 	}
-	return t
+	return t.Underlying()
 }
 
 // If x and y are identical, match returns x.
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index e22b1ff0a0..e077879b9d 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -315,6 +315,7 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 	case *syntax.Operation:
 		if e.Op == syntax.Mul && e.Y == nil {
 			typ := new(Pointer)
+			typ.base = Typ[Invalid] // avoid nil base in invalid recursive type declaration
 			def.setUnderlying(typ)
 			typ.base = check.varType(e.X)
 			// If typ.base is invalid, it's unlikely that *base is particularly