diff --git a/R/importance_sampling.R b/R/importance_sampling.R
index 80ec5c8a..f8dfd4d3 100644
--- a/R/importance_sampling.R
+++ b/R/importance_sampling.R
@@ -114,7 +114,7 @@ assert_importance_sampling_method_is_implemented <- function(x){
     stop("Importance sampling method '",
          x,
          "' is not implemented. Implemented methods: '",
-         paste0(implemented_is_methods, collapse = "', '"),
+         paste0(implemented_is_methods(), collapse = "', '"),
          "'")
   }
 }
diff --git a/tests/testthat/test_tisis.R b/tests/testthat/test_tisis.R
index 44fcc12d..8b3a054a 100644
--- a/tests/testthat/test_tisis.R
+++ b/tests/testthat/test_tisis.R
@@ -224,3 +224,16 @@ test_that("tis_loo and sis_loo are returned", {
   expect_output(print(loo_tis), regexp = "tis_loo")
   expect_output(print(loo_sis), regexp = "sis_loo")
 })
+
+test_that("invalid IS method gives informative error listing valid methods", {
+  LLmat <- example_loglik_matrix()
+  expect_error(
+    loo(LLmat, r_eff = NA, is_method = "bad_method"),
+    "'arg' should be one of"
+  )
+  expect_error(
+    importance_sampling(-LLmat, method = "not_a_method", r_eff = NA),
+    "Implemented methods: 'psis', 'tis', 'sis'",
+    fixed = TRUE
+  )
+})