diff --git a/nnpdf_data/nnpdf_data/commondata/CMS_WCHARM_13TEV/sys_uncertainties.py b/nnpdf_data/nnpdf_data/commondata/CMS_WCHARM_13TEV/sys_uncertainties.py
index a759011699..0d35e707d4 100644
--- a/nnpdf_data/nnpdf_data/commondata/CMS_WCHARM_13TEV/sys_uncertainties.py
+++ b/nnpdf_data/nnpdf_data/commondata/CMS_WCHARM_13TEV/sys_uncertainties.py
@@ -1,4 +1,9 @@
-import numpy as np
+'''
+The full break-down of the systematic uncertainties is not given in the
+HepData format. However, Table 1 of the referenced paper provides the
+different sources of systematic uncertainties bin-by-bin. This table
+is reproduced in the following.
+'''
 
 # Common dict independent of the kinematics
 IND_KIN_DICT = [