diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa7f9607a..34002f238 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 ### Bugfixes
 - [#506](https://github.com/influxdb/telegraf/pull/506): Ping input doesn't return response time metric when timeout. Thanks @titilambert!
 - [#508](https://github.com/influxdb/telegraf/pull/508): Fix prometheus cardinality issue with the `net` plugin
+- [#396](https://github.com/influxdata/telegraf/issues/396): Mongostat diff bug fix
 
 ## v0.10.0 [2016-01-12]
 
diff --git a/plugins/inputs/mongodb/mongostat.go b/plugins/inputs/mongodb/mongostat.go
index e12d797d0..a6ec4cd10 100644
--- a/plugins/inputs/mongodb/mongostat.go
+++ b/plugins/inputs/mongodb/mongostat.go
@@ -390,7 +390,7 @@ func computeLockDiffs(prevLocks, curLocks map[string]LockUsage) []LockUsage {
 func diff(newVal, oldVal, sampleTime int64) int64 {
 	d := newVal - oldVal
 	if d < 0 {
-		d = newVal
+		d = maxVal - oldVal + newVal
 	}
 	return d / sampleTime
 }