diff --git a/packages/istanbul-lib-source-maps/lib/transformer.js b/packages/istanbul-lib-source-maps/lib/transformer.js
index e3042325..3d26c515 100644
--- a/packages/istanbul-lib-source-maps/lib/transformer.js
+++ b/packages/istanbul-lib-source-maps/lib/transformer.js
@@ -119,7 +119,7 @@ class SourceMapTransformer {
             } else {
                 uniqueFiles[getUniqueKey(file)] = {
                     file,
-                    mappedCoverage: fc
+                    mappedCoverage: new MappedCoverage(fc)
                 };
             }
         }