diff --git a/DESCRIPTION b/DESCRIPTION
index d4fac5a1e..a0e8fd465 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,7 +1,7 @@
 Type: Package
 Package: insight
 Title: Easy Access to Model Information for Various Model Objects
-Version: 0.20.2.4
+Version: 0.20.2.5
 Authors@R:
     c(person(given = "Daniel",
              family = "Lüdecke",
@@ -214,3 +214,4 @@ Roxygen: list(markdown = TRUE)
 Config/testthat/edition: 3
 Config/testthat/parallel: true
 Config/Needs/website: easystats/easystatstemplate
+Config/Needs/check: stan-dev/cmdstanr
diff --git a/NEWS.md b/NEWS.md
index 9513aa901..0442975d7 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -16,6 +16,9 @@
   minimum required package version based on the SUGGEST field of the _insight_
   package, instead of the package that was calling the function.
 
+* Fixed issue in `get_modelmatrix()` for models from package *brms* with
+  special functions in the formula (like `mo()`).
+
 # insight 0.20.2
 
 ## New supported models
diff --git a/R/get_modelmatrix.R b/R/get_modelmatrix.R
index 063a05dee..879d7890c 100644
--- a/R/get_modelmatrix.R
+++ b/R/get_modelmatrix.R
@@ -159,7 +159,14 @@ get_modelmatrix.svyglm <- function(x, ...) {
 get_modelmatrix.brmsfit <- function(x, ...) {
   formula_rhs <- safe_deparse(find_formula(x)$conditional[[3]])
   formula_rhs <- stats::as.formula(paste0("~", formula_rhs))
-  .data_in_dots(..., object = formula_rhs, default_data = get_data(x, verbose = FALSE))
+  # the formula used in model.matrix() is not allowed to have special functions,
+  # like brms::mo() and similar. Thus, we reformulate after using "all.vars()",
+  # which will only keep the variable names.
+  .data_in_dots(
+    ...,
+    object = stats::reformulate(all.vars(formula_rhs)),
+    default_data = get_data(x, verbose = FALSE)
+  )
 }
 
 #' @export
diff --git a/tests/testthat/test-brms.R b/tests/testthat/test-brms.R
index fd712b057..2bd985d77 100644
--- a/tests/testthat/test-brms.R
+++ b/tests/testthat/test-brms.R
@@ -2,7 +2,7 @@ skip_on_cran()
 skip_if_offline()
 skip_on_os("mac")
 skip_if_not_installed("brms")
-skip_if_not_installed("httr")
+skip_if_not_installed("httr2")
 
 # Model fitting -----------------------------------------------------------
 
@@ -513,7 +513,6 @@ test_that("find_algorithm", {
   )
 })
 
-
 test_that("get_priors", {
   expect_equal(
     get_priors(m7),
@@ -881,3 +880,12 @@ test_that("clean_parameters", {
     ignore_attr = TRUE
   )
 })
+
+test_that("get_modelmatrix", {
+  out <- get_modelmatrix(m1)
+  expect_identical(dim(out), c(236L, 4L))
+  m9 <- insight::download_model("brms_mo2")
+  skip_if(is.null(m9))
+  out <- get_modelmatrix(m9)
+  expect_identical(dim(out), c(32L, 2L))
+})