diff --git a/packages/metro-transform-worker/src/__tests__/__snapshots__/index-test.js.snap b/packages/metro-transform-worker/src/__tests__/__snapshots__/index-test.js.snap
index 6611d9ac8a..feedb87dc2 100644
--- a/packages/metro-transform-worker/src/__tests__/__snapshots__/index-test.js.snap
+++ b/packages/metro-transform-worker/src/__tests__/__snapshots__/index-test.js.snap
@@ -123,6 +123,10 @@ Array [
     4,
     23,
   ],
+  Array [
+    9,
+    3,
+  ],
 ]
 `;
 
@@ -175,6 +179,10 @@ Array [
     1,
     15,
   ],
+  Array [
+    3,
+    3,
+  ],
 ]
 `;
 
@@ -227,6 +235,10 @@ Array [
     1,
     25,
   ],
+  Array [
+    3,
+    140,
+  ],
 ]
 `;
 
@@ -315,6 +327,10 @@ Array [
     1,
     20,
   ],
+  Array [
+    5,
+    3,
+  ],
 ]
 `;
 
diff --git a/packages/metro-transform-worker/src/__tests__/index-test.js b/packages/metro-transform-worker/src/__tests__/index-test.js
index 40bfcb1d15..6c86699cde 100644
--- a/packages/metro-transform-worker/src/__tests__/index-test.js
+++ b/packages/metro-transform-worker/src/__tests__/index-test.js
@@ -192,7 +192,7 @@ it('transforms an es module with asyncToGenerator', async () => {
 
   expect(result.output[0].type).toBe('js/module');
   expect(result.output[0].data.code).toMatchSnapshot();
-  expect(result.output[0].data.map).toHaveLength(13);
+  expect(result.output[0].data.map).toHaveLength(14);
   expect(result.output[0].data.functionMap).toMatchSnapshot();
   expect(result.dependencies).toEqual([
     {
diff --git a/packages/metro-transform-worker/src/index.js b/packages/metro-transform-worker/src/index.js
index e5f1d55439..c0f2d9e637 100644
--- a/packages/metro-transform-worker/src/index.js
+++ b/packages/metro-transform-worker/src/index.js
@@ -44,7 +44,6 @@ const {
   toSegmentTuple,
 } = require('metro-source-map');
 const metroTransformPlugins = require('metro-transform-plugins');
-const countLines = require('metro/src/lib/countLines');
 const collectDependencies = require('metro/src/ModuleGraph/worker/collectDependencies');
 const {
   InvalidRequireCallError: InternalInvalidRequireCallError,
@@ -441,11 +440,14 @@ async function transformJS(
     ));
   }
 
+  let lineCount;
+  ({lineCount, map} = countLinesAndTerminateMap(code, map));
+
   const output: Array<JsOutput> = [
     {
       data: {
         code,
-        lineCount: countLines(code),
+        lineCount,
         map,
         functionMap: file.functionMap,
       },
@@ -552,9 +554,11 @@ async function transformJSON(
     jsType = 'js/module';
   }
 
+  let lineCount;
+  ({lineCount, map} = countLinesAndTerminateMap(code, map));
   const output: Array<JsOutput> = [
     {
-      data: {code, lineCount: countLines(code), map, functionMap: null},
+      data: {code, lineCount, map, functionMap: null},
       type: jsType,
     },
   ];
@@ -652,6 +656,7 @@ module.exports = {
     const {babelTransformerPath, minifierPath, ...remainingConfig} = config;
 
     const filesKey = getCacheKey([
+      __filename,
       require.resolve(babelTransformerPath),
       require.resolve(minifierPath),
       require.resolve('./utils/getMinifier'),
@@ -670,3 +675,42 @@ module.exports = {
     ].join('$');
   },
 };
+
+function countLinesAndTerminateMap(
+  code: string,
+  map: $ReadOnlyArray<MetroSourceMapSegmentTuple>,
+): {
+  lineCount: number,
+  map: Array<MetroSourceMapSegmentTuple>,
+} {
+  const NEWLINE = /\r\n?|\n|\u2028|\u2029/g;
+  let lineCount = 1;
+  let lastLineStart = 0;
+
+  // Count lines and keep track of where the last line starts
+  for (const match of code.matchAll(NEWLINE)) {
+    lineCount++;
+    lastLineStart = match.index + match[0].length;
+  }
+  const lastLineLength = code.length - lastLineStart;
+  const lastLineIndex1Based = lineCount;
+  const lastLineNextColumn0Based = lastLineLength;
+
+  // If there isn't a mapping at one-past-the-last column of the last line,
+  // add one that maps to nothing. This ensures out-of-bounds lookups hit the
+  // null mapping rather than aliasing to whichever mapping happens to be last.
+  // ASSUMPTION: Mappings are generated in order of increasing line and column.
+  const lastMapping = map[map.length - 1];
+  const terminatingMapping = [lastLineIndex1Based, lastLineNextColumn0Based];
+  if (
+    !lastMapping ||
+    lastMapping[0] !== terminatingMapping[0] ||
+    lastMapping[1] !== terminatingMapping[1]
+  ) {
+    return {
+      lineCount,
+      map: map.concat([terminatingMapping]),
+    };
+  }
+  return {lineCount, map: [...map]};
+}
diff --git a/packages/metro/src/integration_tests/__tests__/__snapshots__/buildGraph-test.js.snap b/packages/metro/src/integration_tests/__tests__/__snapshots__/buildGraph-test.js.snap
index 386f55e8e6..54e826ce1e 100644
--- a/packages/metro/src/integration_tests/__tests__/__snapshots__/buildGraph-test.js.snap
+++ b/packages/metro/src/integration_tests/__tests__/__snapshots__/buildGraph-test.js.snap
@@ -389,6 +389,10 @@ Array [
           20,
           40,
         ],
+        Array [
+          24,
+          3,
+        ],
       ],
     },
     "type": "js/module",