diff --git a/R/antibiogram.R b/R/antibiogram.R
index 0fe11bb71..dc4e707e0 100755
--- a/R/antibiogram.R
+++ b/R/antibiogram.R
@@ -746,7 +746,7 @@ antibiogram.default <- function(x,
       c(1 - conf_interval, 1)
     }
 
-    unique_groups <- unique(wisca_parameters$group)
+    unique_groups <- as.character(unique(wisca_parameters$group))
 
     use_parallel_wisca <- isTRUE(parallel) && n_workers > 1L && length(unique_groups) > 0L
 
@@ -815,6 +815,17 @@ antibiogram.default <- function(x,
         out_wisca$upper_ci[out_wisca$group == group] <- ci_vals[2]
       }
       close(progress)
+      if (isTRUE(info) && simulations >= 500 && length(unique_groups) >= 3) {
+        suggest <- ifelse(.Platform$OS.type == "windows" || in_rstudio(),
+          "plan(multisession)",
+          "plan(multicore)"
+        )
+        if (requireNamespace("future.apply", quietly = TRUE)) {
+          message_("Running in sequential mode. To speed up WISCA, set a parallel {.help [{.fun future::plan}](future::plan)} such as {.code ", suggest, "} and use {.code parallel = TRUE}.")
+        } else {
+          message_("Running in sequential mode. To speed up WISCA, install the {.pkg future.apply} package and then set {.code parallel = TRUE}.")
+        }
+      }
     }
 
     # final output preparation
diff --git a/tests/testthat/test-antibiogram.R b/tests/testthat/test-antibiogram.R
index f7a74f221..6dab925a4 100644
--- a/tests/testthat/test-antibiogram.R
+++ b/tests/testthat/test-antibiogram.R
@@ -152,11 +152,14 @@ test_that("test-antibiogram.R", {
     expect_equal(colnames(wisca_par), colnames(wisca_seq))
     expect_true(isTRUE(attributes(wisca_par)$wisca))
 
-    # 2. coverage values fall within [0, 100] (basic sanity)
+    # 2. coverage values are non-NA and fall within [0, 1]
     ln <- attributes(wisca_par)$long_numeric
-    expect_true(all(ln$coverage >= 0 & ln$coverage <= 1, na.rm = TRUE))
-    expect_true(all(ln$lower_ci <= ln$coverage, na.rm = TRUE))
-    expect_true(all(ln$upper_ci >= ln$coverage, na.rm = TRUE))
+    expect_false(anyNA(ln$coverage))
+    expect_false(anyNA(ln$lower_ci))
+    expect_false(anyNA(ln$upper_ci))
+    expect_true(all(ln$coverage >= 0 & ln$coverage <= 1))
+    expect_true(all(ln$lower_ci <= ln$coverage))
+    expect_true(all(ln$upper_ci >= ln$coverage))
 
     # 3. a second parallel run gives the same column names
     wisca_par2 <- suppressWarnings(suppressMessages(