diff --git a/src/backend/treeify.rs b/src/backend/treeify.rs
index e25b385..22fd3a0 100644
--- a/src/backend/treeify.rs
+++ b/src/backend/treeify.rs
@@ -1,6 +1,7 @@
 //! Treeification: placing some values "under" others if only used
 //! once, to generate more AST-like Wasm code.
 
+use crate::entity::EntityRef;
 use crate::ir::{FunctionBody, Value, ValueDef};
 use crate::Operator;
 use std::collections::{HashMap, HashSet};
@@ -29,6 +30,11 @@ impl Trees {
         for (value, def) in body.values.entries() {
             match def {
                 &ValueDef::Operator(_, ref args, _) => {
+                    // Ignore operators with invalid args: these must
+                    // always be unreachable.
+                    if args.iter().any(|arg| arg.is_invalid()) {
+                        continue;
+                    }
                     // For each of the args, if the value is produced
                     // by a single-output op and is movable, and is
                     // not already recorded in `multi_use`, place it
diff --git a/src/ir/func.rs b/src/ir/func.rs
index 359a0dd..e80ddf3 100644
--- a/src/ir/func.rs
+++ b/src/ir/func.rs
@@ -1,5 +1,5 @@
 use super::{Block, FunctionBodyDisplay, Local, Module, Signature, Type, Value, ValueDef};
-use crate::entity::{EntityVec, PerEntity};
+use crate::entity::{EntityRef, EntityVec, PerEntity};
 
 #[derive(Clone, Debug)]
 pub enum FuncDecl {
@@ -112,6 +112,10 @@ impl FunctionBody {
     }
 
     pub fn resolve_alias(&self, value: Value) -> Value {
+        if value.is_invalid() {
+            return value;
+        }
+
         let mut result = value;
         loop {
             if let &ValueDef::Alias(to) = &self.values[result] {
diff --git a/src/passes/resolve_aliases.rs b/src/passes/resolve_aliases.rs
index 8d254b2..370a7c5 100644
--- a/src/passes/resolve_aliases.rs
+++ b/src/passes/resolve_aliases.rs
@@ -3,6 +3,10 @@
 use crate::{FunctionBody, ValueDef};
 
 pub fn run(body: &mut FunctionBody) {
+    log::debug!(
+        "Resolve aliases: running on:\n{}\n",
+        body.display_verbose("| ")
+    );
     for value in body.values.iter() {
         let mut value_def = std::mem::take(&mut body.values[value]);
         match &mut value_def {