diff --git a/src/uu/tr/src/operation.rs b/src/uu/tr/src/operation.rs
index ae857ef865b..e6eb087486c 100644
--- a/src/uu/tr/src/operation.rs
+++ b/src/uu/tr/src/operation.rs
@@ -238,12 +238,14 @@ impl Sequence {
         set2_uniques.sort();
         set2_uniques.dedup();
 
-        //If the complement flag is used in translate mode, only one unique character may appear in
-        //set2. Validate this with the set of uniques in set2 that we just generated.
+        // If the complement flag is used in translate mode, only one unique
+        // character may appear in set2. Validate this with the set of uniques
+        // in set2 that we just generated.
+        // Also, set2 must not overgrow set1, otherwise the mapping can't be 1:1.
         if set1.iter().any(|x| matches!(x, Self::Class(_)))
             && translating
             && complement_flag
-            && set2_uniques.len() > 1
+            && (set2_uniques.len() > 1 || set2_solved.len() > set1_len)
         {
             return Err(BadSequence::ComplementMoreThanOneUniqueInSet2);
         }
diff --git a/tests/by-util/test_tr.rs b/tests/by-util/test_tr.rs
index 8fe6aab00e6..5d800af947c 100644
--- a/tests/by-util/test_tr.rs
+++ b/tests/by-util/test_tr.rs
@@ -1413,3 +1413,14 @@ fn check_complement_1_unique_in_set2() {
         .args(&["-c", "[:upper:]", arg.as_str()])
         .succeeds();
 }
+
+#[test]
+fn check_complement_set2_too_big() {
+    let x231 = "x".repeat(231);
+
+    // [:upper:] expands to 230 characters, putting more characters in set2
+    // should fail.
+    new_ucmd!()
+        .args(&["-c", "[:upper:]", x231.as_str()])
+        .fails();
+}