diff --git a/.gitignore b/.gitignore
index f374cdc..d9a6697 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,3 +36,4 @@ nosetests.xml
 
 # vim
 *.sw[opqrs]
+*~
diff --git a/pyprophet/data_handling.py b/pyprophet/data_handling.py
index 29c52b7..23ddd65 100644
--- a/pyprophet/data_handling.py
+++ b/pyprophet/data_handling.py
@@ -360,7 +360,7 @@ def filter_(self, idx):
     def add_peak_group_rank(self):
         ids = self.df.tg_num_id.values
         scores = self.df.d_score.values
-        peak_group_ranks = rank(ids, scores)
+        peak_group_ranks = rank(ids, scores.astype(np.float32, copy=False))
         self.df["peak_group_rank"] = peak_group_ranks
 
     @profile