diff --git a/opendrift/models/chemicaldrift.py b/opendrift/models/chemicaldrift.py
index 937048ce2..463ca22db 100644
--- a/opendrift/models/chemicaldrift.py
+++ b/opendrift/models/chemicaldrift.py
@@ -2300,7 +2300,7 @@ def get_chemical_density_array(self, pixelsize_m, z_array,
                 if weight is not None:
                     weights = weight_array[i,:]
                     if origin_marker is not None:
-                        weight_array = weight_array * (originmarker[i,:]==origin_marker)
+                        weight_array[i,:] = weight_array[i,:] * (originmarker[i,:]==origin_marker)
                 else:
                     weights = None
                 for zi in range(len(z_array)-1):