diff --git a/inputs/failing/doublePreInc.ts b/inputs/passing/doublePreInc.ts
similarity index 64%
rename from inputs/failing/doublePreInc.ts
rename to inputs/passing/doublePreInc.ts
index 8187304..c8d66d0 100644
--- a/inputs/failing/doublePreInc.ts
+++ b/inputs/passing/doublePreInc.ts
@@ -1,5 +1,4 @@
-// test_output! 4
-// (This is wrong.)
+// test_output! 3
 
 export default function main() {
   let x = 0;
diff --git a/valuescript_compiler/src/expression_compiler.rs b/valuescript_compiler/src/expression_compiler.rs
index 16e197b..d0990c1 100644
--- a/valuescript_compiler/src/expression_compiler.rs
+++ b/valuescript_compiler/src/expression_compiler.rs
@@ -741,7 +741,19 @@ impl<'a> ExpressionCompiler<'a> {
               }
             }
 
-            reg.clone()
+            let res = self.fnc.allocate_tmp();
+            nested_registers.push(res.clone());
+
+            // Always copy pre-increment value into a new register.
+            // This is a bit heavy-handed (FIXME), but it's consistent with the current policy of
+            // doing this whenever the variable is mutated, which it clearly is. Really though, the
+            // issue is when *other* mutations to this variable occur between now and when it's
+            // inserted.
+            self
+              .fnc
+              .push(Instruction::Mov(Value::Register(reg.clone()), res.clone()));
+
+            res
           }
           TargetAccessor::Nested(nta) => match target_register {
             Some(tr) => {
diff --git a/valuescript_compiler/src/function_compiler.rs b/valuescript_compiler/src/function_compiler.rs
index 88dea08..076e2e6 100644
--- a/valuescript_compiler/src/function_compiler.rs
+++ b/valuescript_compiler/src/function_compiler.rs
@@ -447,7 +447,8 @@ impl FunctionCompiler {
           Some(expr) => {
             let mut expression_compiler = ExpressionCompiler { fnc: self };
 
-            expression_compiler.compile(expr, Some(Register::Return));
+            let compiled = expression_compiler.compile(expr, Some(Register::Return));
+            self.use_(compiled);
           }
         }