diff --git a/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs b/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs
index 33755cfd62c8a..12014b3fd15e9 100644
--- a/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs
+++ b/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs
@@ -547,16 +547,19 @@ private static string EmitIndexOfAnyCustomHelper(string set, Dictionary<string,
                 lines.Add($"internal static int {helperName}(this ReadOnlySpan<char> span)");
                 lines.Add($"{{");
                 int uncheckedStart = lines.Count;
-                lines.Add(excludedAsciiChars.Count == 128 ?
-                          $"    int i = span.IndexOfAnyExceptInRange('\0', '\u007f');" :
+                lines.Add(excludedAsciiChars.Count == 128 ? $"    int i = span.IndexOfAnyExceptInRange('\\0', '\\u007f');" : // no ASCII is in the set
+                          excludedAsciiChars.Count == 0   ? $"    int i = 0;" : // all ASCII is in the set
                           $"    int i = span.IndexOfAnyExcept({EmitSearchValues(excludedAsciiChars.ToArray(), requiredHelpers)});");
                 lines.Add($"    if ((uint)i < (uint)span.Length)");
                 lines.Add($"    {{");
-                lines.Add($"        if (char.IsAscii(span[i]))");
-                lines.Add($"        {{");
-                lines.Add($"            return i;");
-                lines.Add($"        }}");
-                lines.Add($"");
+                if (excludedAsciiChars.Count is not (0 or 128))
+                {
+                    lines.Add($"        if (char.IsAscii(span[i]))");
+                    lines.Add($"        {{");
+                    lines.Add($"            return i;");
+                    lines.Add($"        }}");
+                    lines.Add($"");
+                }
                 if (additionalDeclarations.Count > 0)
                 {
                     lines.AddRange(additionalDeclarations.Select(s => $"        {s}"));