diff --git a/src/chapter_3_2.md b/src/chapter_3_2.md
index 76cf1b1..7384fe4 100644
--- a/src/chapter_3_2.md
+++ b/src/chapter_3_2.md
@@ -178,7 +178,7 @@ pub enum AddressingMode {
 
 impl CPU {
    // ...
-   fn get_operand_address(&self, mode: &AddressingMode) -> u16 {
+   fn get_operand_address(&mut self, mode: &AddressingMode) -> u16 {
 
        match mode {
            AddressingMode::Immediate => self.program_counter,