diff --git a/src/backend/mod.rs b/src/backend/mod.rs
index 7d1cdb2..9b6bcaa 100644
--- a/src/backend/mod.rs
+++ b/src/backend/mod.rs
@@ -153,21 +153,21 @@ impl<'a> WasmFuncBackend<'a> {
                         }
                     }
                 }
-                WasmBlock::BlockParams { from, to } => {
-                    debug_assert_eq!(from.len(), to.len());
-                    for (&from, &(_, to)) in from.iter().zip(to.iter()) {
-                        if self.locals.values[to].is_empty() {
-                            continue;
-                        }
-                        self.lower_value(from, func);
-                    }
-                    for &(_, to) in to.iter().rev() {
-                        if self.locals.values[to].is_empty() {
-                            continue;
-                        }
-                        self.lower_set_value(to, func);
-                    }
-                }
+                // WasmBlock::BlockParams { from, to } => {
+                //     debug_assert_eq!(from.len(), to.len());
+                //     for (&from, &(_, to)) in from.iter().zip(to.iter()) {
+                //         if self.locals.values[to].is_empty() {
+                //             continue;
+                //         }
+                //         self.lower_value(from, func);
+                //     }
+                //     for &(_, to) in to.iter().rev() {
+                //         if self.locals.values[to].is_empty() {
+                //             continue;
+                //         }
+                //         self.lower_set_value(to, func);
+                //     }
+                // }
                 WasmBlock::Return { values } => {
                     for &value in &values[..] {
                         self.lower_value(value, func);
@@ -180,6 +180,31 @@ impl<'a> WasmFuncBackend<'a> {
                     }
                     func.instruction(&wasm_encoder::Instruction::ReturnCall(f.index() as u32));
                 }
+                WasmBlock::BlockParams { from, to } => {
+                    debug_assert_eq!(from.len(), to.len());
+                    for (&from, &(to_ty, to)) in from.iter().zip(to.iter()) {
+                        if self.locals.values[to].is_empty() {
+                            continue;
+                        }
+                        let from_ty = self.body.values[self.body.resolve_alias(from)]
+                            .ty(&self.body.type_pool)
+                            .unwrap();
+                        assert_eq!(from_ty, to_ty);
+                        if self.locals.values[from].len() == 1 {
+                            assert_eq!(from_ty, self.locals.locals[self.locals.values[from][0]]);
+                        }
+                        self.lower_value(from, func);
+                    }
+                    for &(to_ty, to) in to.iter().rev() {
+                        if self.locals.values[to].is_empty() {
+                            continue;
+                        }
+                        if self.locals.values[to].len() == 1 {
+                            assert_eq!(to_ty, self.locals.locals[self.locals.values[to][0]]);
+                        }
+                        self.lower_set_value(to, func);
+                    }
+                }
                 WasmBlock::ReturnCallIndirect { sig, table, values } => {
                     for &value in &values[..] {
                         self.lower_value(value, func);
diff --git a/src/ir/func.rs b/src/ir/func.rs
index ccbea77..7d63fcf 100644
--- a/src/ir/func.rs
+++ b/src/ir/func.rs
@@ -423,8 +423,8 @@ impl FunctionBody {
                 let u = self.resolve_alias(u);
                 if block_inst[u].is_none() {
                     bad.push(format!(
-                        "Use of arg {} at {:?} illegal: not defined",
-                        u, inst
+                        "Use of arg {} at {:?} in {} illegal: not defined",
+                        u, inst, block
                     ));
                     return;
                 }