Moving UV's with a shader (tiling error)

With the following code I tried to animate a texture on a surface by moving its uvs. However it moves, but it doesn't tile correct as the net tile is just grey (stretched from texture's border). Now how do I say that it should display the same texture over nd over again? I also tried TRANSFORM_TEX in the vertex function (same behaviour). Thanks.

  Shader "Custom/test" {
         Properties {
             _Color ("Color", Color) = (1,1,1,1)
             _MainTex ("Albedo (RGB)", 2D) = "white" {}
             _Offset ("Offset", Range(-2.0, 2.0)) = 0.0
         }
         SubShader {
             Tags { "RenderType"="Opaque" }
             LOD 200
             
             CGPROGRAM
             #pragma surface surf Lambert alpha:blend vertex:vert
             #pragma target 3.0
     
             sampler2D _MainTex;
             float _Offset;
     
             struct Input {
                 float2 uv_MainTex;
             };
     
             void vert (inout appdata_full v) {
                 v.texcoord.xy += _Offset;
             }
 
             fixed4 _Color;
     
             void surf (Input IN, inout SurfaceOutput o) {
                 fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
                 o.Albedo = c.rgb;
                 o.Alpha = c.a;
             }
             ENDCG
         }
         FallBack "Diffuse"
     }