diff --git a/R/utils.r b/R/utils.r
index d50a9bf..76397c5 100644
--- a/R/utils.r
+++ b/R/utils.r
@@ -46,7 +46,7 @@ rgen_length <- function(n, x, prob) {
 #' Negative binomial random numbers parametrized
 #' in terms of mean and dispersion coefficient
 #' @param n number of samples to draw
-#' @param mn mean of distribution
+#' @param mn mean of distribution; Must be > 0.
 #' @param disp dispersion coefficient (var/mean); Must be > 1.
 #' @return vector containing the random numbers
 #' @author Flavio Finger
@@ -55,13 +55,13 @@ rgen_length <- function(n, x, prob) {
 #' rnbinom_mean_disp(n = 5, mn = 4, disp = 2)
 rnbinom_mean_disp <- function(n, mn, disp) {
   checkmate::assert_number(
-    n, lower = 0, finite = TRUE, na.ok = FALSE
+    n, lower = 1, finite = TRUE, na.ok = FALSE
   )
   checkmate::assert_number(
     disp, lower = 1, finite = TRUE, na.ok = FALSE
   )
   checkmate::assert_number(
-    mn, lower = 0, finite = TRUE, na.ok = FALSE
+    mn, lower = 1E-100, finite = TRUE, na.ok = FALSE
   )
   size <- mn / (disp - 1)
   stats::rnbinom(n, size = size, mu = mn)
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 235e459..d96e78b 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -8,10 +8,44 @@ test_that("Util functions work", {
 })
 
 test_that("Errors are thrown", {
+  # Checks on 'disp' argument
   expect_error(
     rnbinom_mean_disp(n = 5, mn = 4, disp = 0.9),
     "is not >= 1"
   )
+  expect_error(
+    rnbinom_mean_disp(n = 5, mn = 4, disp = NA),
+    "May not be NA"
+  )
+  expect_error(
+    rnbinom_mean_disp(n = 5, mn = 4, disp = Inf),
+    "Must be finite"
+  )
+  # Checks on 'n' argument
+  expect_error(
+    rnbinom_mean_disp(n = 0, mn = 4, disp = 2),
+    "is not >= 1"
+  )
+  expect_error(
+    rnbinom_mean_disp(n = NA, mn = 4, disp = 2),
+    "May not be NA"
+  )
+  expect_error(
+    rnbinom_mean_disp(n = Inf, mn = 4, disp = 2),
+    "Must be finite"
+  )
+  # Checks on 'mn' argument
+  expect_error(
+    rnbinom_mean_disp(n = 2, mn = 0, disp = 2)
+  )
+  expect_error(
+    rnbinom_mean_disp(n = 2, mn = NA, disp = 2),
+    "May not be NA"
+  )
+  expect_error(
+    rnbinom_mean_disp(n = 2, mn = Inf, disp = 2),
+    "Must be finite"
+  )
 })
 
 test_that("Errors are thrown", {