diff --git a/src/eval/cast.rs b/src/eval/cast.rs
index 92e4212f8..7b3066338 100644
--- a/src/eval/cast.rs
+++ b/src/eval/cast.rs
@@ -7,6 +7,8 @@ use ecow::EcoString;
 
 use super::{Array, Str, Value};
 use crate::diag::StrResult;
+use crate::eval::Type;
+use crate::geom::Length;
 use crate::syntax::Spanned;
 use crate::util::separated_list;
 
@@ -291,6 +293,14 @@ impl CastInfo {
             msg.push_str(", found ");
             msg.push_str(found.type_name());
         }
+        if_chain::if_chain! {
+            if let Value::Int(i) = found;
+            if parts.iter().any(|p| p == Length::TYPE_NAME);
+            if !matching_type;
+            then {
+                msg.push_str(&format!(": a length needs a unit – did you mean {i}pt?"));
+            }
+        };
 
         msg.into()
     }
diff --git a/tests/typ/compiler/hint.typ b/tests/typ/compiler/hint.typ
new file mode 100644
index 000000000..4fd25abb5
--- /dev/null
+++ b/tests/typ/compiler/hint.typ
@@ -0,0 +1,6 @@
+// Test diagnostics.
+// Ref: false
+
+---
+// Error: 1:17-1:19 expected length, found integer: a length needs a unit – did you mean 12pt?
+#set text(size: 12)
\ No newline at end of file