diff --git a/src/lenient_parse/internal/parser.gleam b/src/lenient_parse/internal/parser.gleam
index e79a687..0e4b09c 100644
--- a/src/lenient_parse/internal/parser.gleam
+++ b/src/lenient_parse/internal/parser.gleam
@@ -292,14 +292,16 @@ fn form_base_prefix(
   specifier specifier: String,
   base base: Int,
 ) -> Result(ParseData(Option(#(#(Int, Int), String, Int))), ParseError) {
-  let new_rest = case tokens {
+  let #(start_index, end_index) = index_range
+  let rest = case tokens {
     [] -> []
     [_, ..tokens] -> tokens
   }
+
   Ok(ParseData(
-    data: Some(#(#(index_range.0, index_range.1 + 1), "0" <> specifier, base)),
-    next_index: index_range.1 + 2,
-    tokens: new_rest,
+    data: Some(#(#(start_index, end_index + 1), "0" <> specifier, base)),
+    next_index: end_index + 2,
+    tokens: rest,
   ))
 }