diff --git a/pineappl/src/grid.rs b/pineappl/src/grid.rs
index 6e8f9a99..8abb68b6 100644
--- a/pineappl/src/grid.rs
+++ b/pineappl/src/grid.rs
@@ -1675,12 +1675,14 @@ impl Grid {
 
                         // if `op1` and `op2` below are zero there's no work to do
                         // TODO: ideally we change the for loops instead of vetoing here
-                        if (has_pdf1 && !non_zero_pid_indices
-                            .iter()
-                            .any(|&tuple| tuple == (tgt_pid1_idx, src_pid1_idx)))
-                            || (has_pdf2 && !non_zero_pid_indices
+                        if (has_pdf1
+                            && !non_zero_pid_indices
                                 .iter()
-                                .any(|&tuple| tuple == (tgt_pid2_idx, src_pid2_idx)))
+                                .any(|&tuple| tuple == (tgt_pid1_idx, src_pid1_idx)))
+                            || (has_pdf2
+                                && !non_zero_pid_indices
+                                    .iter()
+                                    .any(|&tuple| tuple == (tgt_pid2_idx, src_pid2_idx)))
                         {
                             continue;
                         }