diff --git a/R/reclin2_pair_ann.R b/R/reclin2_pair_ann.R
index 1e13850..dbcf6fe 100644
--- a/R/reclin2_pair_ann.R
+++ b/R/reclin2_pair_ann.R
@@ -57,14 +57,14 @@ pair_ann <- function(x,
 
   y <- if (deduplication) x else y
 
-  block_result  <- blocking::blocking(x = x[, on],
-                                      y = if (deduplication) NULL else y[, on],
-                                      deduplication = deduplication,
-                                      ...)
-
   x <- data.table::as.data.table(x)
   y <- data.table::as.data.table(y)
 
+  block_result  <- blocking::blocking(x = x[, ..on][[1]],
+                                      y = if (deduplication) NULL else y[, ..on][[1]],
+                                      deduplication = deduplication,
+                                      ...)
+
   a <- x[, ..on]
   a[, `:=`(".x", .I)]
   a <- a[unique(block_result$result[,.(".x"=x, block)]), on = ".x"]