diff --git a/units/src/block.rs b/units/src/block.rs
index 9e8475db4..af4d30b41 100644
--- a/units/src/block.rs
+++ b/units/src/block.rs
@@ -211,10 +211,6 @@ impl ops::Add<BlockInterval> for BlockInterval {
     }
 }
 
-impl ops::AddAssign<BlockInterval> for BlockInterval {
-    fn add_assign(&mut self, rhs: BlockInterval) { self.0 = self.to_u32() + rhs.to_u32(); }
-}
-
 // interval - interval = interval
 impl ops::Sub<BlockInterval> for BlockInterval {
     type Output = BlockInterval;
@@ -225,6 +221,10 @@ impl ops::Sub<BlockInterval> for BlockInterval {
     }
 }
 
+impl ops::AddAssign<BlockInterval> for BlockInterval {
+    fn add_assign(&mut self, rhs: BlockInterval) { self.0 = self.to_u32() + rhs.to_u32(); }
+}
+
 impl ops::SubAssign<BlockInterval> for BlockInterval {
     fn sub_assign(&mut self, rhs: BlockInterval) { self.0 = self.to_u32() - rhs.to_u32(); }
 }