diff --git a/src/runtime/cli/test_command.rs b/src/runtime/cli/test_command.rs
index 577d075d380d..7bc568f313a4 100644
--- a/src/runtime/cli/test_command.rs
+++ b/src/runtime/cli/test_command.rs
@@ -1595,8 +1595,8 @@ impl CommandLineReporter {
         // SAFETY: thread-local Box pinned for the thread; sole `&mut` for the
         // collection loop below (single-threaded CLI report path).
         let map = unsafe { &mut *map.as_ptr() };
-        // `ByteRangeMapping` owns a `MultiArrayList` and is not `Copy`, so
-        // collect mutable borrows into the thread-local map instead — no
+        // `ByteRangeMapping` owns its line table and is not `Copy`, so collect
+        // mutable borrows into the thread-local map instead; there is no
         // double-free risk.
         let mut byte_ranges: Vec<&mut ByteRangeMapping> = Vec::with_capacity(map.len());
         for entry in map.values_mut() {
diff --git a/src/sourcemap_jsc/CodeCoverage.rs b/src/sourcemap_jsc/CodeCoverage.rs
index 966f578b8088..02c9b0974c09 100644
--- a/src/sourcemap_jsc/CodeCoverage.rs
+++ b/src/sourcemap_jsc/CodeCoverage.rs
@@ -5,11 +5,10 @@ use core::ptr::NonNull;
 use bun_ast::Loc;
 use bun_collections::VecExt;
 use bun_collections::bit_set::DynamicBitSet;
-use bun_core::{self, ZigStringSlice};
+use bun_core::{self, ZigStringSlice, strings};
 use bun_jsc::{JSGlobalObject, JSValue, VM, bun_string_jsc};
 use bun_sourcemap::{
     LineOffsetTable, LineOffsetTableColumns as _, Ordinal, ParsedSourceMap, internal_source_map,
-    line_offset_table,
 };
 
 type LinesHits = Vec<u32>;
@@ -417,7 +416,10 @@ pub struct BasicBlockRange {
 }
 
 pub struct ByteRangeMapping {
-    pub(crate) line_offset_table: line_offset_table::List,
+    /// Offset of the start of each line, in UTF-16 code units: the unit JSC
+    /// reports `BasicBlockRange` offsets in. Equal to the byte offset only
+    /// while the source text is pure ASCII.
+    pub(crate) line_starts: Box<[u32]>,
     pub(crate) source_id: i32,
     pub source_url: ZigStringSlice,
 }
@@ -483,7 +485,7 @@ impl ByteRangeMapping {
         function_blocks: &[BasicBlockRange],
         ignore_sourcemap: bool,
     ) -> Result<Report, bun_alloc::AllocError> {
-        let line_starts = self.line_offset_table.items_byte_offset_to_start_of_line();
+        let line_starts = &*self.line_starts;
 
         let mut executable_lines: Bitset;
         let mut lines_which_have_executed: Bitset;
@@ -808,16 +810,42 @@ impl ByteRangeMapping {
         source_id: i32,
         source_url: ZigStringSlice,
     ) -> ByteRangeMapping {
+        let mut line_offset_table = LineOffsetTable::generate(source_contents, 0)
+            .unwrap_or_else(|_| bun_alloc::out_of_memory());
+        let byte_starts = line_offset_table.items_byte_offset_to_start_of_line();
+
+        let line_starts: Box<[u32]> = if strings::is_all_ascii(source_contents) {
+            Box::from(byte_starts)
+        } else {
+            // `source_contents` is the UTF-8 form of the string JSC holds
+            // (8-bit or 16-bit), and JSC's offsets count that string's code
+            // units, so re-measure each line in those.
+            let mut previous_byte_start: usize = 0;
+            let mut code_units: u32 = 0;
+            byte_starts
+                .iter()
+                .map(|&byte_start| {
+                    let byte_start = byte_start as usize;
+                    let line = &source_contents[previous_byte_start..byte_start];
+                    code_units += u32::try_from(strings::element_length_utf8_into_utf16(line))
+                        .expect("int cast");
+                    previous_byte_start = byte_start;
+                    code_units
+                })
+                .collect()
+        };
+        // `MultiArrayList`'s own `Drop` frees the slab only; this drops each
+        // row's `columns_for_non_ascii` box.
+        line_offset_table.drop_elements();
+
         ByteRangeMapping {
-            line_offset_table: LineOffsetTable::generate(source_contents, 0)
-                .unwrap_or_else(|_| bun_alloc::out_of_memory()),
+            line_starts,
             source_id,
             source_url,
         }
     }
 }
 
-// line_offset_table drops automatically.
 // source_url is NOT freed (caller owns it).
 
 #[unsafe(no_mangle)]
diff --git a/test/cli/test/coverage.test.ts b/test/cli/test/coverage.test.ts
index 5e963d9ee366..31f31385aaa0 100644
--- a/test/cli/test/coverage.test.ts
+++ b/test/cli/test/coverage.test.ts
@@ -589,3 +589,87 @@ Ran 1 test across 1 file."
 `);
   expect(result.exitCode).toBe(0);
 });
+
+// JSC reports the offsets of executed and unexecuted blocks in UTF-16 code
+// units of the source text it holds, and the line table those offsets are
+// looked up in was built in bytes, so line attribution drifted after any
+// non-ASCII text. Each non-ASCII variant below has exactly the same number of
+// code units on every line as the ASCII twin (an astral character is two code
+// units and four bytes), so its lcov record must be identical to the twin's;
+// counting bytes or code points shifts it.
+//
+// Both lengths are in UTF-16 code units.
+const coverageBannerUnits = 30;
+const coverageTextUnits = 40;
+function coverageDemo(banner: string, text: string) {
+  return `/*! ${banner} */
+export const raw = String.raw\`${text}\`;
+export const re = /${text}/u;
+export function covered() {
+  return raw.length + re.source.length;
+}
+export function uncovered() {
+  return 1;
+}
+export function alsoUncovered() {
+  return 2;
+}
+`;
+}
+const coverageDemoTest = `
+import { expect, test } from "bun:test";
+import { covered } from "./demo";
+
+test("source text has the expected length", () => {
+  expect(covered()).toBe(${2 * coverageTextUnits});
+});
+`;
+function repeatUnits(piece: string, units: number) {
+  return Buffer.alloc((Buffer.byteLength(piece) * units) / piece.length, piece).toString();
+}
+
+// lcov records of `demo.ts` for each variant, in order.
+async function coverageRecords(variants: Record<string, string>) {
+  using dir = tempDir(
+    "cov-non-ascii",
+    Object.fromEntries(
+      Object.entries(variants).flatMap(([name, source]) => [
+        [`${name}/demo.ts`, source],
+        [`${name}/demo.test.ts`, coverageDemoTest],
+      ]),
+    ),
+  );
+  // `await` here keeps `dir` alive until the processes have finished.
+  return await Promise.all(
+    Object.keys(variants).map(async variant => {
+      const cwd = path.join(String(dir), variant);
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), "test", "--coverage", "--coverage-reporter=lcov", "./demo.test.ts"],
+        cwd,
+        env: bunEnv,
+        stdout: "ignore",
+        stderr: "pipe",
+      });
+      const [stderr, exitCode] = await Promise.all([proc.stderr.text(), proc.exited]);
+      expect(stderr).toContain(" 1 pass\n");
+      expect(exitCode).toBe(0);
+      const record = readFileSync(path.join(cwd, "coverage", "lcov.info"), "utf8")
+        .split("end_of_record")
+        .find(record => record.includes("\nSF:demo.ts\n"));
+      expect(record).toBeDefined();
+      return record!;
+    }),
+  );
+}
+
+test("a non-ASCII preserved comment does not shift coverage lines", async () => {
+  const asciiText = repeatUnits("x", coverageTextUnits);
+  const [ascii, legalComment] = await coverageRecords({
+    ascii: coverageDemo(repeatUnits("x", coverageBannerUnits), asciiText),
+    legalComment: coverageDemo(repeatUnits("©🐰", coverageBannerUnits), asciiText),
+  });
+  // `uncovered` and `alsoUncovered` start on lines 7 and 10.
+  expect(ascii).toContain("\nDA:7,0\n");
+  expect(ascii).toContain("\nDA:10,0\n");
+  expect(legalComment).toBe(ascii);
+});