diff --git a/packages/babel-core/src/transformation/file/merge-map.js b/packages/babel-core/src/transformation/file/merge-map.js
index 991373862e1e..ff2e65b042b8 100644
--- a/packages/babel-core/src/transformation/file/merge-map.js
+++ b/packages/babel-core/src/transformation/file/merge-map.js
@@ -94,7 +94,7 @@ export default function mergeSourceMap(
 }
 
 function makeMappingKey(item: { line: number, columnStart: number }) {
-  return JSON.stringify([item.line, item.columnStart]);
+  return `${item.line}/${item.columnStart}`;
 }
 
 function eachOverlappingGeneratedOutputRange(