diff --git a/statannot/statannot.py b/statannot/statannot.py
index 05e3009..2c618f0 100644
--- a/statannot/statannot.py
+++ b/statannot/statannot.py
@@ -468,10 +468,10 @@ def get_box_data(box_plotter, boxName):
     # Build the list of box data structure pairs
     box_struct_pairs = []
     for i_box_pair, (box1, box2) in enumerate(box_pairs):
-        valid = box1 in box_names and box2 in box_names
-        if not valid:
-            raise ValueError("box_pairs contains an invalid box pair.")
-            pass
+
+        if box1 not in box_names or box2 not in box_names:
+            raise ValueError(f"box_pairs contains an invalid box pair, expected {box1 if not box1 in box_names else box2} in {box_names}")
+
         # i_box_pair will keep track of the original order of the box pairs.
         box_struct1 = dict(box_structs_dic[box1], i_box_pair=i_box_pair)
         box_struct2 = dict(box_structs_dic[box2], i_box_pair=i_box_pair)