diff --git a/libbeat/common/mapstr.go b/libbeat/common/mapstr.go
index 7c44e2d5ac3..d70f05dd349 100644
--- a/libbeat/common/mapstr.go
+++ b/libbeat/common/mapstr.go
@@ -96,10 +96,6 @@ func (m MapStr) deepUpdateMap(d MapStr, overwrite bool) {
 }
 
 func deepUpdateValue(old interface{}, val MapStr, overwrite bool) interface{} {
-	if old == nil {
-		return val
-	}
-
 	switch sub := old.(type) {
 	case MapStr:
 		if sub == nil {
@@ -117,7 +113,9 @@ func deepUpdateValue(old interface{}, val MapStr, overwrite bool) interface{} {
 		tmp.deepUpdateMap(val, overwrite)
 		return tmp
 	default:
-		// This should never happen
+		// We reach the default branch if old is no map or if old == nil.
+		// In either case we return `val`, such that the old value is completely
+		// replaced when merging.
 		return val
 	}
 }