diff --git a/R/confint.R b/R/confint.R
index 3061beb4f..fd5356151 100644
--- a/R/confint.R
+++ b/R/confint.R
@@ -39,7 +39,7 @@ confint.rxSolve <- function(object, parm = NULL, level = 0.95, ...) {
   .useT <- TRUE
   if (any(names(.args) == "useT")) {
     .useT <- .args$useT
-    checkmate::assertLogical(useT, len=1, any.missing=FALSE, .var.name="pred")
+    checkmate::assertLogical(.useT, len=1, any.missing=FALSE, .var.name="useT")
   }
   if (any(names(.args) == "mean")) {
     .mean <- .args$mean
diff --git a/tests/testthat/test-meanProb.R b/tests/testthat/test-meanProb.R
index 247ea9dae..aa1ffc95a 100644
--- a/tests/testthat/test-meanProb.R
+++ b/tests/testthat/test-meanProb.R
@@ -89,4 +89,28 @@ test_that("test meanProb()", {
 
   expect_equal(ci1$eff, ci2$eff)
 
+
+  ci1 <- confint(pk1, "blood", mean=TRUE, useT=FALSE)
+
+  # use dplyr
+  ci2 <- pk1 |>
+    dplyr::group_by(time) |>
+    dplyr::reframe(eff=meanProbs(blood, c(0.025, 0.5, 0.975), na.rm=TRUE, names=FALSE, useT=FALSE),
+                   p1=c(0.025, 0.5, 0.975), Percentile=c("2.5%", "50%", "97.5%")) |>
+    dplyr::arrange(time, p1)
+
+  expect_equal(ci1$eff, ci2$eff)
+
+  ci1 <- confint(pk1, "blood", mean=TRUE, useT=FALSE, pred=TRUE)
+
+  # use dplyr
+  ci2 <- pk1 |>
+    dplyr::group_by(time) |>
+    dplyr::reframe(eff=meanProbs(blood, c(0.025, 0.5, 0.975), na.rm=TRUE, names=FALSE, useT=FALSE,
+                                 pred=TRUE),
+                   p1=c(0.025, 0.5, 0.975), Percentile=c("2.5%", "50%", "97.5%")) |>
+    dplyr::arrange(time, p1)
+
+  expect_equal(ci1$eff, ci2$eff)
+
 })