diff --git a/src/ExCSS/Model/ValueBuilder.cs b/src/ExCSS/Model/ValueBuilder.cs
index 38fc69e..3f40c18 100644
--- a/src/ExCSS/Model/ValueBuilder.cs
+++ b/src/ExCSS/Model/ValueBuilder.cs
@@ -44,7 +44,7 @@ public void Apply(Token token)
                     Add(token);
                     break;
                 case TokenType.Whitespace:
-                    if (_values.Count > 0 && IsSlash(_values[^1]) == false)
+                    if (_values.Count > 0 && IsSlash(_values[_values.Count - 1]) == false)
                         _buffer = token;
                     break;
                 case TokenType.Dimension: