diff --git a/dust-lang/src/compiler.rs b/dust-lang/src/compiler.rs
index d6dbe1a..8314966 100644
--- a/dust-lang/src/compiler.rs
+++ b/dust-lang/src/compiler.rs
@@ -632,7 +632,15 @@ impl<'src> Compiler<'src> {
                 .push((right_instruction, right_type, right_position));
         }
 
-        let destination = Destination::Register(self.next_register());
+        let destination = if is_assignment {
+            match left {
+                Argument::Register(register) => Destination::Register(register),
+                Argument::Local(local_index) => Destination::Local(local_index),
+                Argument::Constant(_) => Destination::Register(self.next_register()),
+            }
+        } else {
+            Destination::Register(self.next_register())
+        };
         let instruction = match operator {
             Token::Plus | Token::PlusEqual => Instruction::add(destination, left, right),
             Token::Minus | Token::MinusEqual => Instruction::subtract(destination, left, right),
@@ -1268,6 +1276,7 @@ impl<'src> Compiler<'src> {
                 should_return_value,
             });
 
+            self.update_return_type(previous_expression_type)?;
             self.emit_instruction(r#return, Type::None, self.current_position);
         }
 
diff --git a/dust-lang/tests/math.rs b/dust-lang/tests/math.rs
index 150214f..f17fe8c 100644
--- a/dust-lang/tests/math.rs
+++ b/dust-lang/tests/math.rs
@@ -59,11 +59,11 @@ fn add_assign() {
                 ),
                 (
                     Instruction::add(
-                        Destination::Register(0),
-                        Argument::Register(0),
+                        Destination::Local(0),
+                        Argument::Local(0),
                         Argument::Constant(2)
                     ),
-                    Type::Integer,
+                    Type::None,
                     Span(17, 19)
                 ),
                 (
@@ -176,11 +176,11 @@ fn divide_assign() {
                 ),
                 (
                     Instruction::divide(
-                        Destination::Register(0),
-                        Argument::Register(0),
+                        Destination::Local(0),
+                        Argument::Local(0),
                         Argument::Constant(0)
                     ),
-                    Type::Integer,
+                    Type::None,
                     Span(17, 19)
                 ),
                 (
@@ -339,11 +339,11 @@ fn multiply_assign() {
                 ),
                 (
                     Instruction::multiply(
-                        Destination::Register(0),
-                        Argument::Register(0),
+                        Destination::Local(0),
+                        Argument::Local(0),
                         Argument::Constant(2)
                     ),
-                    Type::Integer,
+                    Type::None,
                     Span(17, 19)
                 ),
                 (
@@ -440,11 +440,11 @@ fn subtract_assign() {
                 ),
                 (
                     Instruction::subtract(
-                        Destination::Register(0),
-                        Argument::Register(0),
+                        Destination::Local(0),
+                        Argument::Local(0),
                         Argument::Constant(2)
                     ),
-                    Type::Integer,
+                    Type::None,
                     Span(18, 20)
                 ),
                 (