diff --git a/internal/loop/run.go b/internal/loop/run.go
index d5b471ddf..1e5ba5520 100644
--- a/internal/loop/run.go
+++ b/internal/loop/run.go
@@ -28,6 +28,7 @@ func CurrentFPS() float64 {
 }
 
 func IsRunning() bool {
+	// TODO: Need lock?
 	if currentRunContext == nil {
 		return false
 	}
diff --git a/run.go b/run.go
index 994b0cbf5..fe5d3ab25 100644
--- a/run.go
+++ b/run.go
@@ -15,6 +15,7 @@
 package ebiten
 
 import (
+	"math"
 	"sync/atomic"
 
 	"github.com/hajimehoshi/ebiten/internal/loop"
@@ -143,7 +144,12 @@ func SetScreenScale(scale float64) {
 
 // ScreenScale returns the current screen scale.
 //
+// If Run is not called, this returns NaN.
+//
 // This function is concurrent-safe.
 func ScreenScale() float64 {
+	if !loop.IsRunning() {
+		return math.NaN()
+	}
 	return ui.ScreenScale()
 }