diff --git a/pyrefly/lib/alt/attr.rs b/pyrefly/lib/alt/attr.rs
index bcd88bc272..f63e168a36 100644
--- a/pyrefly/lib/alt/attr.rs
+++ b/pyrefly/lib/alt/attr.rs
@@ -27,7 +27,6 @@ use ruff_python_ast::name::Name;
 use ruff_text_size::TextRange;
 use starlark_map::small_set::SmallSet;
 use vec1::Vec1;
-use vec1::vec1;
 
 use crate::alt::answers::LookupAnswer;
 use crate::alt::answers_solver::AnswersSolver;
@@ -614,7 +613,8 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         } else if !error_messages.is_empty() {
             error_messages.sort();
             error_messages.dedup();
-            let mut msg = vec1![error_messages.join("\n")];
+            let header = error_messages.remove(0);
+            let mut details = error_messages;
             // Skip suggestions when we have a partial union failure to avoid suggesting
             // attributes from the types that have them when the problem is that some types
             // don't have the attribute at all.
@@ -623,9 +623,8 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     .as_ref()
                     .and_then(|attr_base| self.suggest_attribute_name(attr_name, attr_base))
             {
-                msg.push(format!("Did you mean `{suggestion}`?"));
+                details.push(format!("Did you mean `{suggestion}`?"));
             }
-            let (header, details) = msg.split_off_first();
             errors
                 .error_builder(range, ErrorKind::MissingAttribute, header)
                 .with_details(details)
diff --git a/pyrefly/lib/error/error.rs b/pyrefly/lib/error/error.rs
index c6292c1229..f66634a3ea 100644
--- a/pyrefly/lib/error/error.rs
+++ b/pyrefly/lib/error/error.rs
@@ -348,6 +348,10 @@ impl Error {
         error_kind: ErrorKind,
     ) -> Self {
         let display_range = module.display_range(range);
+        assert!(
+            !header.contains(['\n', '\r']),
+            "error header must not contain newlines"
+        );
         let msg_header = header.into_boxed_str();
         let msg_details = if details.is_empty() {
             None
@@ -585,6 +589,23 @@ mod tests {
         );
     }
 
+    #[test]
+    #[should_panic(expected = "error header must not contain newlines")]
+    fn test_error_header_rejects_newlines() {
+        let module_info = Module::new(
+            ModuleName::from_str("test"),
+            ModulePath::filesystem(PathBuf::from("test.py")),
+            Arc::new("x = 1".to_owned()),
+        );
+        let _ = Error::new(
+            module_info,
+            TextRange::new(TextSize::new(0), TextSize::new(1)),
+            "first line\r\nsecond line".to_owned(),
+            Vec::new(),
+            ErrorKind::BadReturn,
+        );
+    }
+
     /// Integration test: verify that binary operator errors from the type checker
     /// produce secondary annotations labeling both operands with their types.
     #[test]