diff --git a/vignettes/stackgbm.bib b/vignettes/stackgbm.bib
index 189d14e..4970fed 100644
--- a/vignettes/stackgbm.bib
+++ b/vignettes/stackgbm.bib
@@ -1,7 +1,7 @@
 @inproceedings{chen2016xgboost,
   title        = {{XGBoost}: A scalable tree boosting system},
   author       = {Chen, Tianqi and Guestrin, Carlos},
-  booktitle    = {Proceedings of the 22nd acm sigkdd international conference on knowledge discovery and data mining},
+  booktitle    = {Proceedings of the 22nd {ACM} {SIGKDD} international conference on knowledge discovery and data mining},
   pages        = {785--794},
   year         = {2016},
   organization = {ACM}