diff --git a/cli/tests/cli_run.rs b/cli/tests/cli_run.rs
index b3d98098c8..b2d90626ff 100644
--- a/cli/tests/cli_run.rs
+++ b/cli/tests/cli_run.rs
@@ -178,7 +178,7 @@ mod cli_run {
             &example_file("benchmarks", "NQueens.roc"),
             "nqueens",
             &[],
-            "724\n",
+            "4\n",
             false,
         );
     }
@@ -219,6 +219,18 @@ mod cli_run {
         );
     }
 
+    #[test]
+    #[serial(deriv)]
+    fn run_rbtree_delete_not_optimized() {
+        check_output(
+            &example_file("benchmarks", "RBTreeDel.roc"),
+            "rbtree-del",
+            &[],
+            "30\n",
+            false,
+        );
+    }
+
     //    #[test]
     //    #[serial(effect)]
     //    fn run_effect_unoptimized() {
diff --git a/compiler/can/src/def.rs b/compiler/can/src/def.rs
index c082971270..82c9fb71c8 100644
--- a/compiler/can/src/def.rs
+++ b/compiler/can/src/def.rs
@@ -789,13 +789,19 @@ fn canonicalize_pending_def<'a>(
 
             let arity = typ.arity();
 
-            let def_symbol = match &loc_can_pattern.value {
-                Pattern::Identifier(symbol) => *symbol,
-                _ => panic!("standalone annotations must be on identifiers"),
+            let problem = match &loc_can_pattern.value {
+                Pattern::Identifier(symbol) => RuntimeError::NoImplementationNamed {
+                    def_symbol: *symbol,
+                },
+                Pattern::Shadowed(region, loc_ident) => RuntimeError::Shadowing {
+                    original_region: *region,
+                    shadow: loc_ident.clone(),
+                },
+                _ => RuntimeError::NoImplementation,
             };
 
             // Fabricate a body for this annotation, that will error at runtime
-            let value = Expr::RuntimeError(RuntimeError::NoImplementation(def_symbol));
+            let value = Expr::RuntimeError(problem);
             let is_closure = arity > 0;
             let loc_can_expr = if !is_closure {
                 Located {
diff --git a/compiler/problem/src/can.rs b/compiler/problem/src/can.rs
index 2cb107b922..1173630fb3 100644
--- a/compiler/problem/src/can.rs
+++ b/compiler/problem/src/can.rs
@@ -145,7 +145,10 @@ pub enum RuntimeError {
     InvalidUnicodeCodePoint(Region),
 
     /// When the author specifies a type annotation but no implementation
-    NoImplementation(Symbol),
+    NoImplementationNamed {
+        def_symbol: Symbol,
+    },
+    NoImplementation,
 
     /// cases where the `[]` value (or equivalently, `forall a. a`) pops up
     VoidValue,
diff --git a/compiler/reporting/src/error/canonicalize.rs b/compiler/reporting/src/error/canonicalize.rs
index f1b1281d08..8c12467eb0 100644
--- a/compiler/reporting/src/error/canonicalize.rs
+++ b/compiler/reporting/src/error/canonicalize.rs
@@ -635,7 +635,7 @@ fn pretty_runtime_error<'b>(
                 region
             );
         }
-        RuntimeError::NoImplementation(_) => todo!("no implementation, unreachable"),
+        RuntimeError::NoImplementation | RuntimeError::NoImplementationNamed { .. }  => todo!("no implementation, unreachable"),
         RuntimeError::NonExhaustivePattern => {
             unreachable!("not currently reported (but can blow up at runtime)")
         }
diff --git a/compiler/reporting/tests/test_reporting.rs b/compiler/reporting/tests/test_reporting.rs
index 5b58ebe4d2..aeaf14b1d8 100644
--- a/compiler/reporting/tests/test_reporting.rs
+++ b/compiler/reporting/tests/test_reporting.rs
@@ -1129,7 +1129,7 @@ mod test_reporting {
                 3│>      when True is
                 4│>          _ -> 3.14
 
-                This `when`expression produces:
+                This `when` expression produces:
 
                     Float a
 
diff --git a/examples/benchmarks/RBTreeDel.roc b/examples/benchmarks/RBTreeDel.roc
index 745d26c9d1..31cf63b2fa 100644
--- a/examples/benchmarks/RBTreeDel.roc
+++ b/examples/benchmarks/RBTreeDel.roc
@@ -14,14 +14,18 @@ ConsList a : [ Nil, Cons a (ConsList a) ]
 
 main : Task.Task {} []
 main =
-    m = makeMap 33
+    # benchmarks use 4_200_000
+    m = makeMap 420
+
+    val = fold (\_, v, r -> if v then r + 1 else r) m 0
 
-    # val = fold (\_, v, r -> if v then r + 1 else r) m 0
-    val = depth m
     val
         |> Str.fromInt
         |> Task.putLine
 
+boom : Str -> a
+boom = \_ -> boom ""
+
 makeMap : I64 -> Map
 makeMap = \n ->
     makeMapHelp n n Leaf
@@ -154,11 +158,13 @@ rebalanceLeft = \c, l, k, v, r ->
   when l is
       Node Black _ _ _ _   -> Del (balanceLeft (setRed l) k v r) (isBlack c)
       Node Red lx kx vx rx -> Del (Node Black lx kx vx (balanceLeft (setRed rx) k v r)) False
+      _ -> boom "unreachable"
 
 rebalanceRight  = \c, l, k, v, r ->
   when r is
       Node Black _ _ _ _   -> Del (balanceRight l k v (setRed r))  (isBlack c)
       Node Red lx kx vx rx -> Del (Node Black (balanceRight l k v (setRed lx)) kx vx rx) False
+      _ -> boom "unreachable"