diff --git a/pbcabi/binaryreader.py b/pbcabi/binaryreader.py
index 2c2153c..bbe81e8 100644
--- a/pbcabi/binaryreader.py
+++ b/pbcabi/binaryreader.py
@@ -26,6 +26,12 @@ class BinaryReader:
         self.size = len(buf)
         self.position = 0
 
+    def __repr__(self) -> str:
+        return 'BinaryReader[{} / {}]'.format(self.position, self.size)
+
+    def __str__(self) -> str:
+        return repr(self)
+
     def readBytes(self, num_bytes: int) -> bytes:
         '''
         Reads a number of bytes from stream.
@@ -86,9 +92,9 @@ class BinaryReader:
         '''
         # TODO: Test!
         result = self.readUIntBigEndian(num_bytes)
-        half = 2**(num_bytes * 8)
-        if result > half:
-            result -= half
+        full = 2**(num_bytes * 8)
+        if result >= full//2:
+            result -= full
         return result
 
     def readUIntLittleEndian(self, num_bytes: int) -> int:
@@ -107,9 +113,9 @@ class BinaryReader:
         '''
         # TODO: Test!
         result = self.readUIntLittleEndian(num_bytes)
-        half = 2**(num_bytes * 8)
-        if result > half:
-            result -= half
+        full = 2**(num_bytes * 8)
+        if result >= full//2:
+            result -= full
         return result
 
     def readLeb128(self) -> int:
diff --git a/pbcabi/data.py b/pbcabi/data.py
index 6b0cd17..67ee7e5 100644
--- a/pbcabi/data.py
+++ b/pbcabi/data.py
@@ -95,9 +95,7 @@ class BlockchainAddress(ByteData):
         return repr(self)
 
     def shard_id(self, num_shards: int) -> str:
-        reader = BinaryReader(self.data)
-        reader.readBytes(17)
-        shard = reader.readUInt32BigEndian()
+        shard = BinaryReader(self.data[17:]).readSignedIntBigEndian(4)
         shard = abs(shard) % num_shards
         return f'Shard{shard}'