diff --git a/R/alert_functions.R b/R/alert_functions.R
index ebce96f..0f6d60e 100644
--- a/R/alert_functions.R
+++ b/R/alert_functions.R
@@ -349,6 +349,14 @@ pipe_infodengue <- function(cities, cid10="A90", datarelatorio, finalday = NULL,
 
 .as_legacy_alert_objects <- function(result) {
       data <- result$data
+      diseases <- unique(c(as.character(data$CID10),
+                           as.character(result$parameters$cid10)))
+      if (!"CID10" %in% names(data) ||
+          !"cid10" %in% names(result$parameters) ||
+          length(diseases) != 1L || anyNA(diseases) || any(!nzchar(diseases))) {
+            stop("The legacy adapter requires exactly one disease in data and parameters.",
+                 call. = FALSE)
+      }
       groups <- split(seq_len(nrow(data)), factor(data$cidade,
                                                    levels = unique(data$cidade)))
       legacy <- lapply(names(groups), function(city) {
diff --git a/tests/testthat/test-legacy-disease-guard.R b/tests/testthat/test-legacy-disease-guard.R
new file mode 100644
index 0000000..77cd0e8
--- /dev/null
+++ b/tests/testthat/test-legacy-disease-guard.R
@@ -0,0 +1,10 @@
+test_that("legacy adapter rejects mixed or inconsistent disease results", {
+  result <- review_result()
+  mixed <- result
+  mixed$data$CID10[1] <- "A92.0"
+  expect_error(AlertTools:::.as_legacy_alert_objects(mixed), "exactly one disease")
+  inconsistent <- result
+  inconsistent$parameters$cid10 <- "A92.0"
+  expect_error(AlertTools:::.as_legacy_alert_objects(inconsistent), "exactly one disease")
+  expect_s3_class(AlertTools:::.as_legacy_alert_objects(result)[[1]], "alerta")
+})