diff --git a/compiler/internal/typeparams/collect_test.go b/compiler/internal/typeparams/collect_test.go
index 26791ea59..3960bb04b 100644
--- a/compiler/internal/typeparams/collect_test.go
+++ b/compiler/internal/typeparams/collect_test.go
@@ -98,7 +98,11 @@ func TestVisitor(t *testing.T) {
 	info, pkg := f.Check("pkg/test", file)
 
 	lookupObj := func(name string) types.Object {
-		return srctesting.LookupObj(pkg, name)
+		obj := srctesting.LookupObj(pkg, name)
+		if obj == nil {
+			t.Fatalf("Object %q not found", name)
+		}
+		return obj
 	}
 	lookupType := func(name string) types.Type { return lookupObj(name).Type() }
 	lookupDecl := func(name string) ast.Node {
diff --git a/compiler/internal/typeparams/instance.go b/compiler/internal/typeparams/instance.go
index 10e0df69f..9ccb82abe 100644
--- a/compiler/internal/typeparams/instance.go
+++ b/compiler/internal/typeparams/instance.go
@@ -3,6 +3,7 @@ package typeparams
 import (
 	"fmt"
 	"go/types"
+	"sort"
 	"strings"
 
 	"github.com/gopherjs/gopherjs/compiler/internal/symbol"
@@ -270,3 +271,23 @@ func (i PackageInstanceSets) Add(instances ...Instance) {
 func (i PackageInstanceSets) ID(inst Instance) int {
 	return i.Pkg(inst.Object.Pkg()).ID(inst)
 }
+
+func (i PackageInstanceSets) String() string {
+	pkgName := make([]string, 0, len(i))
+	for pkg := range i {
+		pkgName = append(pkgName, pkg)
+	}
+	sort.Strings(pkgName)
+	buf := strings.Builder{}
+	for _, pkg := range pkgName {
+		buf.WriteString(pkg)
+		buf.WriteString(":\n")
+		iset := i[pkg]
+		for _, inst := range iset.values {
+			buf.WriteString("\t")
+			buf.WriteString(inst.String())
+			buf.WriteString("\n")
+		}
+	}
+	return buf.String()
+}
diff --git a/compiler/internal/typeparams/map.go b/compiler/internal/typeparams/map.go
index 7edbdc016..13cf24a30 100644
--- a/compiler/internal/typeparams/map.go
+++ b/compiler/internal/typeparams/map.go
@@ -174,7 +174,7 @@ func (im *InstanceMap[V]) Keys() []Instance {
 func (im *InstanceMap[V]) String() string {
 	entries := make([]string, 0, im.Len())
 	im.Iterate(func(key Instance, value V) {
-		entries = append(entries, fmt.Sprintf("%v:%v", key, value))
+		entries = append(entries, fmt.Sprintf("%v:%v", key.String(), value))
 	})
 	sort.Strings(entries)
 	return `{` + strings.Join(entries, `, `) + `}`