Question Sprite Shader Cutoff bug

Hi I have to create a shader to have a fill effect on a sprite renderer.
I have this:

Code (CSharp):

1. Shader "Sprites/DefaultCutoff"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
6.     _Color("Tint", Color) = (0,0,0,0)
7.         _TransitionTex("Transition Texture", 2D) = "white" {}
8.     _Cutoff("Cutoff", Range(0, 1)) = 0
9.         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
10.     }
11.  
12.         SubShader
13.     {
14.         Tags
15.     {
16.         "Queue" = "Transparent"
17.         "IgnoreProjector" = "True"
18.         "RenderType" = "Transparent"
19.         "PreviewType" = "Plane"
20.         "CanUseSpriteAtlas" = "True"
21.     }
22.  
23.         Cull Off
24.         Lighting Off
25.         ZWrite Off
26.         //Blend SrcAlpha OneMinusSrcAlpha
27.         Blend SrcAlpha OneMinusSrcAlpha
28.  
29.         Pass
30.     {
31.         CGPROGRAM
32. #pragma vertex vert
33. #pragma fragment frag
34. #pragma multi_compile _ PIXELSNAP_ON
35. #include "UnityCG.cginc"
36.  
37.         struct appdata_t
38.     {
39.         float4 vertex   : POSITION;
40.         float4 color    : COLOR;
41.         float2 texcoord : TEXCOORD0;
42.     };
43.  
44.     struct v2f
45.     {
46.         float4 vertex   : SV_POSITION;
47.         fixed4 color : COLOR;
48.         float2 texcoord  : TEXCOORD0;
49.     };
50.  
51.     fixed4 _Color;
52.     float _Cutoff;
53.     sampler2D _TransitionTex;
54.  
55.     v2f vert(appdata_t IN)
56.     {
57.         v2f OUT;
58.         OUT.vertex = UnityObjectToClipPos(IN.vertex);
59.         OUT.texcoord = IN.texcoord;
60.         OUT.color = IN.color * _Color;
61. #ifdef PIXELSNAP_ON
62.         OUT.vertex = UnityPixelSnap(OUT.vertex);
63. #endif
64.  
65.         return OUT;
66.     }
67.  
68.     sampler2D _MainTex;
69.     sampler2D _AlphaTex;
70.     float _AlphaSplitEnabled;
71.  
72.     fixed4 SampleSpriteTexture(float2 uv)
73.     {
74.         fixed4 color = tex2D(_MainTex, uv);
75.  
76. #if UNITY_TEXTURE_ALPHASPLIT_ALLOWED
77.         if (_AlphaSplitEnabled)
78.             color.a = tex2D(_AlphaTex, uv).r;
79. #endif //UNITY_TEXTURE_ALPHASPLIT_ALLOWED
80.  
81.         return color;
82.     }
83.  
84.     fixed4 frag(v2f IN) : SV_Target
85.     {
86.         fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
87.     c.rgb *= c.a;
88.  
89.     fixed4 transit = tex2D(_TransitionTex, IN.texcoord);
90.     if (transit.b > _Cutoff)
91.         c.a = 0;
92.  
93.     return c;
94.     }
95.         ENDCG
96.     }
97.     }
98. }

I get a very annoing line on the top of the sprite when the cutoff is more than 0.5....


How to solve this?