diff --git a/libse/LibSE.csproj b/libse/LibSE.csproj
index 60cd22bbf..a03361d11 100644
--- a/libse/LibSE.csproj
+++ b/libse/LibSE.csproj
@@ -46,6 +46,7 @@
     <Compile Include="BluRaySup\PaletteInfo.cs" />
     <Compile Include="BluRaySup\ToolBox.cs" />
     <Compile Include="BmpReader.cs" />
+    <Compile Include="CharUtils.cs" />
     <Compile Include="Configuration.cs" />
     <Compile Include="ContainerFormats\AviRiffData.cs" />
     <Compile Include="ContainerFormats\Ebml\Element.cs" />
diff --git a/libse/SubtitleFormats/Chk.cs b/libse/SubtitleFormats/Chk.cs
index 621255959..06b3a8907 100644
--- a/libse/SubtitleFormats/Chk.cs
+++ b/libse/SubtitleFormats/Chk.cs
@@ -143,7 +143,7 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
                 if (end - start > 0)
                     text = _codePage.GetString(buffer, start, end - start);
             }
-            if (text.Length > 4 && text[0] == 0x1f && text[1] == 'R' && text[4] == '.' && "0123456789".Contains(text[2]) && "0123456789".Contains(text[3]))
+            if (text.Length > 4 && text[0] == 0x1f && text[1] == 'R' && text[4] == '.' && CharUtils.IsDigit(text[2]) && CharUtils.IsDigit(text[3]))
             {
                 text = text.Remove(0, 5);
             }