diff --git a/tests/testthat/test-derive_vars_transposed.R b/tests/testthat/test-derive_vars_transposed.R
index 3126af5b27..c7ddcb2b73 100644
--- a/tests/testthat/test-derive_vars_transposed.R
+++ b/tests/testthat/test-derive_vars_transposed.R
@@ -170,3 +170,97 @@ test_that("derive_vars_transposed Test 5: ATC variables are merged properly", {
     keys = c("STUDYID", "USUBJID", "CMDECOD", "ATC4CD")
   )
 })
+
+## Test 6: `Relationship` argument handled by left_join  ----
+test_that("derive_vars_transposed Test 6: left_join throws error when argument specified by
+          `relationship` is incorrect", {
+  cm <- tibble::tribble(
+    ~STUDYID,  ~USUBJID,       ~CMGRPID, ~CMREFID,  ~CMDECOD,
+    "STUDY01", "BP40257-1001", "14",     "1192056", "PARACETAMOL",
+    "STUDY01", "BP40257-1001", "18",     "2007001", "SOLUMEDROL",
+    "STUDY01", "BP40257-1002", "19",     "2791596", "SPIRONOLACTONE"
+  )
+  facm <- tibble::tribble(
+    ~STUDYID,  ~USUBJID,       ~FAGRPID, ~FAREFID,  ~FATESTCD,  ~FASTRESC,
+    "STUDY01", "BP40257-1001", "1",      "1192056", "CMATC1CD", "N",
+    "STUDY01", "BP40257-1001", "1",      "1192056", "CMATC2CD", "N02",
+    "STUDY01", "BP40257-1001", "1",      "1192056", "CMATC3CD", "N02B",
+    "STUDY01", "BP40257-1001", "1",      "1192056", "CMATC4CD", "N02BE",
+    "STUDY01", "BP40257-1001", "1",      "2007001", "CMATC1CD", "D",
+    "STUDY01", "BP40257-1001", "1",      "2007001", "CMATC2CD", "D10",
+    "STUDY01", "BP40257-1001", "1",      "2007001", "CMATC3CD", "D10A",
+    "STUDY01", "BP40257-1001", "1",      "2007001", "CMATC4CD", "D10AA",
+    "STUDY01", "BP40257-1001", "2",      "2007001", "CMATC1CD", "D",
+    "STUDY01", "BP40257-1001", "2",      "2007001", "CMATC2CD", "D07",
+    "STUDY01", "BP40257-1001", "2",      "2007001", "CMATC3CD", "D07A",
+    "STUDY01", "BP40257-1001", "2",      "2007001", "CMATC4CD", "D07AA",
+    "STUDY01", "BP40257-1001", "3",      "2007001", "CMATC1CD", "H",
+    "STUDY01", "BP40257-1001", "3",      "2007001", "CMATC2CD", "H02",
+    "STUDY01", "BP40257-1001", "3",      "2007001", "CMATC3CD", "H02A",
+    "STUDY01", "BP40257-1001", "3",      "2007001", "CMATC4CD", "H02AB",
+    "STUDY01", "BP40257-1002", "1",      "2791596", "CMATC1CD", "C",
+    "STUDY01", "BP40257-1002", "1",      "2791596", "CMATC2CD", "C03",
+    "STUDY01", "BP40257-1002", "1",      "2791596", "CMATC3CD", "C03D",
+    "STUDY01", "BP40257-1002", "1",      "2791596", "CMATC4CD", "C03DA"
+  )
+
+  dup <- tibble(
+    STUDYID = "STUDYID01",
+    USUBJID = "BP40257-1001",
+    FAGRPID = "1",
+    FAREFID = "1192056"
+  )
+
+  facm1 <- bind_rows(facm, dup)
+
+  expect_no_error(
+    cm %>%
+      derive_vars_transposed(
+        facm,
+        by_vars = exprs(USUBJID, CMREFID = FAREFID),
+        id_vars = exprs(FAGRPID),
+        key_var = FATESTCD,
+        value_var = FASTRESC,
+        relationship = "one-to-many"
+      ) %>%
+      select(USUBJID, CMDECOD, starts_with("CMATC"))
+  )
+
+  expect_error(
+    cm %>%
+      derive_vars_transposed(
+        facm,
+        by_vars = exprs(USUBJID, CMREFID = FAREFID),
+        id_vars = exprs(FAGRPID),
+        key_var = FATESTCD,
+        value_var = FASTRESC,
+        relationship = "one-to-one"
+      ) %>%
+      select(USUBJID, CMDECOD, starts_with("CMATC"))
+  )
+  expect_error(
+    cm %>%
+      derive_vars_transposed(
+        facm,
+        by_vars = exprs(USUBJID, CMREFID = FAREFID),
+        id_vars = exprs(FAGRPID),
+        key_var = FATESTCD,
+        value_var = FASTRESC,
+        relationship = "many-to-one"
+      ) %>%
+      select(USUBJID, CMDECOD, starts_with("CMATC"))
+  )
+
+  expect_error(
+    cm %>%
+      derive_vars_transposed(
+        facm1,
+        by_vars = exprs(USUBJID, CMREFID = FAREFID),
+        id_vars = exprs(FAGRPID),
+        key_var = FATESTCD,
+        value_var = FASTRESC,
+        relationship = "one-to-one"
+      ) %>%
+      select(USUBJID, CMDECOD, starts_with("CMATC"))
+  )
+})