diff --git a/lib/io/src/i2c.rs b/lib/io/src/i2c.rs
index 211936e..d462e23 100644
--- a/lib/io/src/i2c.rs
+++ b/lib/io/src/i2c.rs
@@ -33,7 +33,7 @@ pub mod mock_i2c {
     use heapless::FnvIndexMap;
 
     /// A fixed-size map of I2C values, indexed by device address and register address
-    type I2cValues = FnvIndexMap<(u8, u16), Option<u8>, 40>;
+    type I2cValues = FnvIndexMap<(u8, u16), Option<u8>, 36>;
 
     /// A mock I2C instance which can be used for testing
     pub struct MockI2c {