2D Sprite Flash Shader - unwanted outline

Dear all,

I am trying to flash my sprite to completely white to make a hitting effect on monster. I have searched on some post and found that a really good shader to get this effect.

However, I found that this shader will give a small outline to the sprite in Unity 5, please help to figure what is the problem.

I am an idiot of shader :( please help.

the outline:

when flashed:

The shader:

 // Compiled shader for Android, uncompressed size: 3.5KB
 
 // Skipping shader variants that would not be included into build of current scene.
 
 Shader "Sprites/DefaultColorFlash" {
 Properties {
 [PerRendererData]  _MainTex ("Sprite Texture", 2D) = "white" { }
  _Color ("Tint", Color) = (1,1,1,1)
  _FlashColor ("Flash Color", Color) = (1,1,1,1)
  _FlashAmount ("Flash Amount", Range(0,1)) = 0
 [MaterialToggle]  PixelSnap ("Pixel snap", Float) = 0
 }
 SubShader { 
  Tags { "QUEUE"="Transparent" "IGNOREPROJECTOR"="true" "RenderType"="Transparent" "PreviewType"="Plane" "CanUseSpriteAtlas"="true" }
 
 
  // Stats for Vertex shader:
  //        gles : 3 math, 1 texture
  Pass {
   Tags { "QUEUE"="Transparent" "IGNOREPROJECTOR"="true" "RenderType"="Transparent" "PreviewType"="Plane" "CanUseSpriteAtlas"="true" }
   ZWrite Off
   Cull Off
   Fog { Mode Off }
   Blend SrcAlpha OneMinusSrcAlpha
   GpuProgramID 31062
 Program "vp" {
 SubProgram "gles " {
 // Stats: 3 math, 1 textures
 Keywords { "DUMMY" }
 "!!GLES
 
 
 #ifdef VERTEX
 
 attribute vec4 _glesVertex;
 attribute vec4 _glesColor;
 attribute vec4 _glesMultiTexCoord0;
 uniform highp mat4 glstate_matrix_mvp;
 uniform lowp vec4 _Color;
 varying lowp vec4 xlv_COLOR;
 varying mediump vec2 xlv_TEXCOORD0;
 void main ()
 {
   highp vec2 tmpvar_1;
   tmpvar_1 = _glesMultiTexCoord0.xy;
   lowp vec4 tmpvar_2;
   mediump vec2 tmpvar_3;
   tmpvar_3 = tmpvar_1;
   highp vec4 tmpvar_4;
   tmpvar_4 = (_glesColor * _Color);
   tmpvar_2 = tmpvar_4;
   gl_Position = (glstate_matrix_mvp * _glesVertex);
   xlv_COLOR = tmpvar_2;
   xlv_TEXCOORD0 = tmpvar_3;
 }
 
 
 
 #endif
 #ifdef FRAGMENT
 
 uniform lowp vec4 _FlashColor;
 uniform highp float _FlashAmount;
 uniform sampler2D _MainTex;
 varying lowp vec4 xlv_COLOR;
 varying mediump vec2 xlv_TEXCOORD0;
 void main ()
 {
   lowp vec4 c_1;
   lowp vec4 tmpvar_2;
   tmpvar_2 = (texture2D (_MainTex, xlv_TEXCOORD0) * xlv_COLOR);
   c_1.w = tmpvar_2.w;
   highp vec3 tmpvar_3;
   tmpvar_3 = mix (tmpvar_2.xyz, _FlashColor.xyz, vec3(_FlashAmount));
   c_1.xyz = tmpvar_3;
   c_1.xyz = (c_1.xyz * tmpvar_2.w);
   gl_FragData[0] = c_1;
 }
 
 
 
 #endif"
 }
 SubProgram "gles " {
 // Stats: 3 math, 1 textures
 Keywords { "PIXELSNAP_ON" }
 "!!GLES
 
 
 #ifdef VERTEX
 
 attribute vec4 _glesVertex;
 attribute vec4 _glesColor;
 attribute vec4 _glesMultiTexCoord0;
 uniform highp vec4 _ScreenParams;
 uniform highp mat4 glstate_matrix_mvp;
 uniform lowp vec4 _Color;
 varying lowp vec4 xlv_COLOR;fixed
 varying mediump vec2 xlv_TEXCOORD0;
 void main ()
 {
   highp vec2 tmpvar_1;
   tmpvar_1 = _glesMultiTexCoord0.xy;
   lowp vec4 tmpvar_2;
   mediump vec2 tmpvar_3;
   highp vec4 tmpvar_4;
   tmpvar_4 = (glstate_matrix_mvp * _glesVertex);
   tmpvar_3 = tmpvar_1;
   highp vec4 tmpvar_5;
   tmpvar_5 = (_glesColor * _Color);
   tmpvar_2 = tmpvar_5;
   highp vec4 pos_6;
   pos_6.zw = tmpvar_4.zw;
   highp vec2 tmpvar_7;
   tmpvar_7 = (_ScreenParams.xy * 0.5);
   pos_6.xy = ((floor(
     (((tmpvar_4.xy / tmpvar_4.w) * tmpvar_7) + vec2(0.5, 0.5))
   ) / tmpvar_7) * tmpvar_4.w);
   gl_Position = pos_6;
   xlv_COLOR = tmpvar_2;
   xlv_TEXCOORD0 = tmpvar_3;
 }
 
 
 
 #endif
 #ifdef FRAGMENT
 
 uniform lowp vec4 _FlashColor;
 uniform highp float _FlashAmount;
 uniform sampler2D _MainTex;
 varying lowp vec4 xlv_COLOR;
 varying mediump vec2 xlv_TEXCOORD0;
 void main ()
 {
   lowp vec4 c_1;
   lowp vec4 tmpvar_2;
   tmpvar_2 = (texture2D (_MainTex, xlv_TEXCOORD0) * xlv_COLOR);
   c_1.w = tmpvar_2.w;
   highp vec3 tmpvar_3;
   tmpvar_3 = mix (tmpvar_2.xyz, _FlashColor.xyz, vec3(_FlashAmount));
   c_1.xyz = tmpvar_3;
   c_1.xyz = (c_1.xyz * tmpvar_2.w);
   gl_FragData[0] = c_1;
 }
 
 
 
 #endif"
 }
 }
 Program "fp" {
 SubProgram "gles " {
 Keywords { "DUMMY" }
 "!!GLES"
 }
 SubProgram "gles " {
 Keywords { "PIXELSNAP_ON" }
 "!!GLES"
 }
 }
  }
 }
 }