diff --git a/R/qp_dilute.R b/R/qp_dilute.R
index 0d827c7..7daec27 100644
--- a/R/qp_dilute.R
+++ b/R/qp_dilute.R
@@ -29,12 +29,12 @@ qp_dilute.data.frame <- function(x, target_conc = NULL, target_vol = 15,
   if (remove_standards)
     x <- x |> dplyr::filter(.data$sample_type != "standard")
 
-  if (!length_is_recyclable(target_conc, x)) {
+  if (!length_is_recyclable(length(target_conc), x)) {
     rlang::abort("`target_conc` length is not 1 or nrow(x)")
   }
 
   conc_col_name <- ifelse(
-    ".mean_pred_conc" %in% colnames(x), ".mean_pred_conc", ".pred_conc"
+    has_cols(x, ".mean_pred_conc"), ".mean_pred_conc", ".pred_conc"
   )
 
   if (is.null(target_conc)) {
@@ -64,5 +64,5 @@ qp_dilute.list <- function(x, target_conc = NULL, target_vol = 15,
 }
 
 length_is_recyclable <- function(n, x) {
-  is.null(n) || n == 1 || n == nrow(x)
+  n == 0 || n == 1 || n == nrow(x)
 }
diff --git a/tests/testthat/test-qp_dilute.R b/tests/testthat/test-qp_dilute.R
new file mode 100644
index 0000000..e8c7269
--- /dev/null
+++ b/tests/testthat/test-qp_dilute.R
@@ -0,0 +1,38 @@
+test_that("multiplication works", {
+  expect_equal(2 * 2, 4)
+})
+
+test_that("length_is_recyclable works", {
+  expect_true(length_is_recyclable(1, iris[1, ]))
+  expect_true(length_is_recyclable(1, iris[1:2, ]))
+  expect_true(length_is_recyclable(2, iris[1:2, ]))
+  expect_true(length_is_recyclable(0, iris[1:2, ]))
+  expect_false(length_is_recyclable(2, iris))
+})
+
+test_that("dilution prefers .mean_pred_conc", {
+  data <- data.frame(
+    .pred_conc = c(1, 2),
+    .mean_pred_conc = c(5, 10)
+  )
+  out <- qp_dilute(data, target_conc = 5, target_vol = 2)
+  expect_equal(out$sample_to_add, c(2, 1))
+  expect_equal(out$add_to, c(0, 1))
+})
+
+test_that("dilution can use .pred_conc", {
+  data <- data.frame(.pred_conc = c(1, 2))
+  out <- qp_dilute(data, target_conc = 1, target_vol = 2)
+  expect_equal(out$sample_to_add, c(2, 1))
+  expect_equal(out$add_to, c(0, 1))
+})
+
+test_that("NULL target_conc uses lowest concentration", {
+  data <- data.frame(
+    sample_type = c("standard", "standard", "unknown", "unknown"),
+    .pred_conc = 1:4
+  )
+  out <- qp_dilute(data, target_vol = 2)
+  expect_equal(out$sample_to_add, c(6, 3, 2, 1.5))
+  expect_equal(out$add_to, c(-4, -1, 0, 0.5))
+})