diff --git a/ethcore/src/migrations/state/v7.rs b/ethcore/src/migrations/state/v7.rs
index 547339b45..cf27c9362 100644
--- a/ethcore/src/migrations/state/v7.rs
+++ b/ethcore/src/migrations/state/v7.rs
@@ -12,17 +12,33 @@ impl SimpleMigration for ToV7 {
 	}
 
 	fn simple_migrate(&self, mut key: Vec<u8>, value: Vec<u8>) -> Option<(Vec<u8>, Vec<u8>)> {
+		if key.len() != 32 {
+			// metadata key, ignore.
+			return Some((key, value));
+		}
+
 		let val_hash = value.sha3();
-		assert!(key.len() == 32); // all keys in the state db are hashes.
 		let key_h = H256::from_slice(&key[..]);
 		if key_h != val_hash {
 			// this is a key which has been xor'd with an address.
-			// recover the address
+			// recover the address.
 			let address = key_h ^ val_hash;
+
+			// check that the address is actually a 20-byte value.
+			// the leftmost 12 bytes should be zero.
+			if &address[0..12] != &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] {
+				// metadata key that was 32 bytes, ignore.
+				return Some((key, value));
+			}
+
 			let address_hash = address.sha3();
 
-			let new_key = address_hash ^ val_hash;
-			key.copy_from_slice(&new_key[..]);
+			// create the xor'd key in place.
+			key.copy_from_slice(&*val_hash);
+			let last_dst: &[u8] = &*address_hash;
+			for (k, a) in key[12..].iter_mut().zip(&last_dst[12..]) {
+				*k ^= *a;
+			}
 		}
 		// nothing to do here
 		Some((key, value))