diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index 2272ac0645..d48dcbffc3 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -540,6 +540,10 @@ func (check *Checker) interfaceType(iface *Interface, ityp *ast.InterfaceType, d
 		}
 		iface.embeddeds = append(iface.embeddeds, named)
 		// collect embedded methods
+		if debug && embed.allMethods == nil {
+			check.dump("%s: incomplete embedded interface %s", pos, named)
+			unreachable()
+		}
 		for _, m := range embed.allMethods {
 			if check.declareInSet(&mset, pos, m) {
 				iface.allMethods = append(iface.allMethods, m)
@@ -579,7 +583,11 @@ func (check *Checker) interfaceType(iface *Interface, ityp *ast.InterfaceType, d
 	// claim source order in the future. Revisit.
 	sort.Sort(byUniqueTypeName(iface.embeddeds))
 
-	sort.Sort(byUniqueMethodName(iface.allMethods))
+	if iface.allMethods == nil {
+		iface.allMethods = make([]*Func, 0) // mark interface as complete
+	} else {
+		sort.Sort(byUniqueMethodName(iface.allMethods))
+	}
 }
 
 // byUniqueTypeName named type lists can be sorted by their unique type names.