diff --git a/lib/coverage.js b/lib/coverage.js
index 6513cbe..5b9a109 100644
--- a/lib/coverage.js
+++ b/lib/coverage.js
@@ -67,6 +67,7 @@ function coverage(data, val) {
 }
 
 function populateCoverage(cov) {
+  var linesLen;
   var results = {};
 
   results.LOC = 0;
@@ -93,12 +94,15 @@ function populateCoverage(cov) {
         source: null
       };
 
-      if (file_stats.lines == null) {
+      if (file_stats.lines === null) {
         file_stats.lines = file.lines;
       }
       else {
-        for (var i = 0; i < file.lines; i++) {
-          if (file.lines[i]) file_stats.lines[i] += file.lines[i];
+        linesLen = file.lines.length;
+        for (var i = 0; i < linesLen; i++) {
+          if (file.lines[i] !== null) {
+            file_stats.lines[i] += file.lines[i];
+          }
         }
       }
 
@@ -107,7 +111,7 @@ function populateCoverage(cov) {
       }
 
       results.files[name] = file_stats;
-    }
+      }
   }
 
   /* Calculate statistics */