diff --git a/shader.go b/shader.go
index 79a869994..0ee67dfa3 100644
--- a/shader.go
+++ b/shader.go
@@ -24,6 +24,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/graphics"
 	"github.com/hajimehoshi/ebiten/internal/mipmap"
 	"github.com/hajimehoshi/ebiten/internal/shader"
+	"github.com/hajimehoshi/ebiten/internal/shaderir"
 )
 
 var shaderSuffix string
@@ -105,6 +106,7 @@ func __vertex(position vec2, texCoord vec2, color vec4) (vec4, vec2, vec4) {
 type Shader struct {
 	shader       *mipmap.Shader
 	uniformNames []string
+	uniformTypes []shaderir.Type
 }
 
 func NewShader(src []byte) (*Shader, error) {
@@ -137,6 +139,7 @@ func NewShader(src []byte) (*Shader, error) {
 	return &Shader{
 		shader:       mipmap.NewShader(s),
 		uniformNames: s.UniformNames,
+		uniformTypes: s.Uniforms,
 	}, nil
 }
 
@@ -157,15 +160,67 @@ func (s *Shader) convertUniforms(uniforms map[string]interface{}) []interface{}
 	}
 
 	us := make([]interface{}, len(names))
-	for n, u := range uniforms {
-		idx, ok := names[n]
-		if !ok {
-			// TODO: Panic here?
+	for name, idx := range names {
+		if v, ok := uniforms[name]; ok {
+			// TODO: Check the uniform variable types?
+			us[idx] = v
 			continue
 		}
-		us[idx] = u
+
+		t := s.uniformTypes[idx]
+		v := zeroUniformValue(t)
+		if v == nil {
+			panic(fmt.Sprintf("ebiten: unexpected uniform variable type: %s", t.String()))
+		}
+		us[idx] = v
 	}
 
-	// TODO: Check the uniform variable types?
+	// TODO: Panic if uniforms include an invalid name
+
 	return us
 }
+
+func zeroUniformValue(t shaderir.Type) interface{} {
+	switch t.Main {
+	case shaderir.Bool:
+		return false
+	case shaderir.Int:
+		return 0
+	case shaderir.Float:
+		return float32(0)
+	case shaderir.Vec2:
+		return make([]float32, 2)
+	case shaderir.Vec3:
+		return make([]float32, 3)
+	case shaderir.Vec4:
+		return make([]float32, 4)
+	case shaderir.Mat2:
+		return make([]float32, 4)
+	case shaderir.Mat3:
+		return make([]float32, 9)
+	case shaderir.Mat4:
+		return make([]float32, 16)
+	case shaderir.Array:
+		switch t.Sub[0].Main {
+		case shaderir.Bool:
+			return make([]bool, t.Length)
+		case shaderir.Int:
+			return make([]int, t.Length)
+		case shaderir.Float:
+			return make([]float32, t.Length)
+		case shaderir.Vec2:
+			return make([]float32, t.Length*2)
+		case shaderir.Vec3:
+			return make([]float32, t.Length*3)
+		case shaderir.Vec4:
+			return make([]float32, t.Length*4)
+		case shaderir.Mat2:
+			return make([]float32, t.Length*4)
+		case shaderir.Mat3:
+			return make([]float32, t.Length*9)
+		case shaderir.Mat4:
+			return make([]float32, t.Length*16)
+		}
+	}
+	return nil
+}
diff --git a/shader_test.go b/shader_test.go
index c2b8f4400..139047975 100644
--- a/shader_test.go
+++ b/shader_test.go
@@ -419,3 +419,32 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+func TestShaderUninitializedUniformVariables(t *testing.T) {
+	const w, h = 16, 16
+
+	dst, _ := NewImage(w, h, FilterDefault)
+	s, err := NewShader([]byte(`package main
+
+var U vec4
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	return U
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w, h, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			var want color.RGBA
+			if got != want {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}