diff --git a/fuzz/fuzz_targets/roundtrip_roundtrip.rs b/fuzz/fuzz_targets/roundtrip_roundtrip.rs
index 20cdb64..7beb722 100644
--- a/fuzz/fuzz_targets/roundtrip_roundtrip.rs
+++ b/fuzz/fuzz_targets/roundtrip_roundtrip.rs
@@ -9,6 +9,9 @@ fuzz_target!(|module: wasm_smith::Module| {
     let orig_bytes = module.to_bytes();
     let parsed_module = Module::from_wasm_bytes(&orig_bytes[..]).unwrap();
     let roundtrip_bytes = parsed_module.to_wasm_bytes();
+    if let Ok(filename) = std::env::var("ROUNDTRIP_WASM_SAVE") {
+        std::fs::write(filename, &roundtrip_bytes[..]).unwrap();
+    }
     let parsed_roundtrip_module = Module::from_wasm_bytes(&roundtrip_bytes[..]).unwrap();
     let _ = parsed_roundtrip_module.to_wasm_bytes();
 });
diff --git a/src/backend/final.rs b/src/backend/final.rs
index 296d4a3..d3eeb60 100644
--- a/src/backend/final.rs
+++ b/src/backend/final.rs
@@ -26,6 +26,21 @@ impl<'a, FT: FuncTypeSink> WasmContext<'a, FT> {
         self.func_type_sink.add_signature(params, results)
     }
 
+    fn find_fallthrough_ty<'b>(
+        &mut self,
+        params: &[Type],
+        mut targets: impl Iterator<Item = &'b SerializedBlockTarget>,
+    ) -> u32 {
+        let fallthrough_rets = targets
+            .find_map(|target| match target {
+                SerializedBlockTarget::Fallthrough(tys, ..) => Some(&tys[..]),
+                _ => None,
+            })
+            .unwrap_or(&[]);
+
+        self.create_type(params.to_vec(), fallthrough_rets.to_vec())
+    }
+
     fn translate(&mut self, op: &SerializedOperator, locations: &Locations) {
         log::trace!("translate: {:?}", op);
         match op {
@@ -69,9 +84,13 @@ impl<'a, FT: FuncTypeSink> WasmContext<'a, FT> {
                 ref if_true,
                 ref if_false,
             } => {
-                self.wasm.operators.push(wasm_encoder::Instruction::If(
-                    wasm_encoder::BlockType::Empty,
-                ));
+                let fallthrough_ty =
+                    self.find_fallthrough_ty(&[], [if_true, if_false].iter().map(|&targ| targ));
+                self.wasm
+                    .operators
+                    .push(wasm_encoder::Instruction::If(BlockType::FunctionType(
+                        fallthrough_ty,
+                    )));
                 self.translate_target(1, if_true, locations);
                 self.wasm.operators.push(wasm_encoder::Instruction::Else);
                 self.translate_target(1, if_false, locations);
@@ -81,10 +100,17 @@ impl<'a, FT: FuncTypeSink> WasmContext<'a, FT> {
                 ref targets,
                 ref default,
             } => {
-                let ty = self.create_type(vec![Type::I32], vec![]);
-                for _ in 0..(targets.len() + 2) {
+                let fallthrough_ty = self.find_fallthrough_ty(
+                    &[Type::I32],
+                    targets.iter().chain(std::iter::once(default)),
+                );
+                self.wasm.operators.push(wasm_encoder::Instruction::Block(
+                    wasm_encoder::BlockType::FunctionType(fallthrough_ty),
+                ));
+                let index_ty = self.create_type(vec![Type::I32], vec![]);
+                for _ in 0..(targets.len() + 1) {
                     self.wasm.operators.push(wasm_encoder::Instruction::Block(
-                        wasm_encoder::BlockType::FunctionType(ty),
+                        wasm_encoder::BlockType::FunctionType(index_ty),
                     ));
                 }
 
@@ -132,12 +158,17 @@ impl<'a, FT: FuncTypeSink> WasmContext<'a, FT> {
     ) {
         log::trace!("translate_target: {:?}", target);
         match target {
-            &SerializedBlockTarget::Fallthrough(ref ops) => {
+            &SerializedBlockTarget::Fallthrough(_, ref ops) => {
                 for op in ops {
                     self.translate(op, locations);
                 }
+                if extra_blocks > 0 {
+                    self.wasm
+                        .operators
+                        .push(wasm_encoder::Instruction::Br((extra_blocks - 1) as u32));
+                }
             }
-            &SerializedBlockTarget::Branch(branch, ref ops) => {
+            &SerializedBlockTarget::Branch(branch, _, ref ops) => {
                 for op in ops {
                     self.translate(op, locations);
                 }
diff --git a/src/backend/serialize.rs b/src/backend/serialize.rs
index a223d0a..c940cab 100644
--- a/src/backend/serialize.rs
+++ b/src/backend/serialize.rs
@@ -12,6 +12,7 @@ use crate::{
     ValueDef,
 };
 use fxhash::FxHashSet;
+use wasmparser::Type;
 
 /// A Wasm function body with a serialized sequence of operators that
 /// mirror Wasm opcodes in every way *except* for locals corresponding
@@ -24,21 +25,21 @@ pub struct SerializedBody {
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum SerializedBlockTarget {
-    Fallthrough(Vec<SerializedOperator>),
-    Branch(usize, Vec<SerializedOperator>),
+    Fallthrough(Vec<Type>, Vec<SerializedOperator>),
+    Branch(usize, Vec<Type>, Vec<SerializedOperator>),
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum SerializedOperator {
     StartBlock {
         header: BlockId,
-        params: Vec<(wasmparser::Type, Value)>,
-        results: Vec<wasmparser::Type>,
+        params: Vec<(Type, Value)>,
+        results: Vec<Type>,
     },
     StartLoop {
         header: BlockId,
-        params: Vec<(wasmparser::Type, Value)>,
-        results: Vec<wasmparser::Type>,
+        params: Vec<(Type, Value)>,
+        results: Vec<Type>,
     },
     Br(SerializedBlockTarget),
     BrIf {
@@ -119,8 +120,8 @@ impl SerializedBlockTarget {
         w: &mut W,
     ) {
         match self {
-            &SerializedBlockTarget::Branch(_, ref ops)
-            | &SerializedBlockTarget::Fallthrough(ref ops) => {
+            &SerializedBlockTarget::Branch(_, _, ref ops)
+            | &SerializedBlockTarget::Fallthrough(_, ref ops) => {
                 for op in ops {
                     op.visit_value_locals(r, w);
                 }
@@ -246,10 +247,15 @@ impl<'a> SerializedBodyContext<'a> {
                             self.push_value(value, &mut rev_ops);
                         }
                         rev_ops.reverse();
+                        let tys: Vec<Type> = self.f.blocks[target.target]
+                            .params
+                            .iter()
+                            .map(|&(ty, _)| ty)
+                            .collect();
                         log::trace!(" -> ops: {:?}", rev_ops);
                         match target.relative_branch {
-                            Some(branch) => SerializedBlockTarget::Branch(branch, rev_ops),
-                            None => SerializedBlockTarget::Fallthrough(rev_ops),
+                            Some(branch) => SerializedBlockTarget::Branch(branch, tys, rev_ops),
+                            None => SerializedBlockTarget::Fallthrough(tys, rev_ops),
                         }
                     })
                     .collect::<Vec<_>>();
@@ -283,8 +289,6 @@ impl<'a> SerializedBodyContext<'a> {
                         self.operators.extend(rev_ops);
                         self.operators
                             .push(SerializedOperator::BrTable { targets, default });
-                        self.operators
-                            .push(SerializedOperator::Operator(Operator::Unreachable));
                     }
                     &Terminator::Return { ref values, .. } => {
                         let mut rev_ops = vec![];