diff --git a/DESCRIPTION b/DESCRIPTION
index 01f91fcb9a..da78c46c3d 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -70,7 +70,7 @@ Suggests:
     rpart,
     sf (>= 0.7-3),
     svglite (>= 2.1.2),
-    testthat (>= 3.1.5),
+    testthat (>= 3.3.0),
     tibble,
     vdiffr (>= 1.0.6),
     xml2
diff --git a/R/utilities.R b/R/utilities.R
index 598b9a8fc3..2aa6ab5b42 100644
--- a/R/utilities.R
+++ b/R/utilities.R
@@ -927,7 +927,7 @@ compute_data_size <- function(data, size, default = 0.9,
   panels <- arg_match0(panels, c("across", "by", "ignore"))
 
   if (panels == "across") {
-    res <- split(data[[var]], data$PANEL, drop = FALSE)
+    res <- split(data[[var]], data$PANEL, drop = TRUE)
     res <- vapply(res, resolution, FUN.VALUE = numeric(1), ...)
     res <- min(res, na.rm = TRUE)
   } else if (panels == "by") {
diff --git a/tests/testthat/test-utilities.R b/tests/testthat/test-utilities.R
index 66e380c973..33fb9fb5cc 100644
--- a/tests/testthat/test-utilities.R
+++ b/tests/testthat/test-utilities.R
@@ -197,3 +197,13 @@ test_that("fallback_palette finds palettes", {
   pal <- fallback_palette(sc)
   expect_true(is_discrete_pal(pal))
 })
+
+test_that("compute_data_size handles gnarly cases", {
+  # Test missing levels
+  df <- data_frame0(
+    x = seq(0, 20, by = 2),
+    PANEL = factor(rep("B", 11), levels = c("A", "B"))
+  )
+  new <- compute_data_size(df, size = NULL, target = "width", default = 1)
+  expect_all_equal(new$width, 2)
+})