diff --git a/src/CoverageTransformer.js b/src/CoverageTransformer.js
index 927279f..2aa2c5e 100644
--- a/src/CoverageTransformer.js
+++ b/src/CoverageTransformer.js
@@ -140,7 +140,7 @@ export default class CoverageTransformer {
 			origSourceFilename = rawSourceMap.sources[0];
 
 			if (origSourceFilename && path.extname(origSourceFilename) !== '' && rawSourceMap.sources.length === 1) {
-				origFileName = rawSourceMap.file;
+				origFileName = rawSourceMap.file || rawSourceMap.sources[0];
 				fileName = filePath.replace(path.extname(origFileName), path.extname(origSourceFilename));
 				rawSourceMap.file = fileName;
 				rawSourceMap.sources = [fileName];