diff --git a/lib/file.js b/lib/file.js
index 0cfb8a4b..3cb0ce26 100644
--- a/lib/file.js
+++ b/lib/file.js
@@ -250,6 +250,10 @@ FileCoverage.prototype.merge = function (other) {
         var i,
             retArray = that.data.b[k],
             secondArray = other.b[k];
+        if (!retArray) {
+            that.data.b[k] = secondArray;
+            return;
+        }
         for (i = 0; i < retArray.length; i += 1) {
             retArray[i] += secondArray[i];
         }