diff --git a/adafruit_register/i2c_bit.py b/adafruit_register/i2c_bit.py
index 581be5d..92a327c 100644
--- a/adafruit_register/i2c_bit.py
+++ b/adafruit_register/i2c_bit.py
@@ -43,10 +43,10 @@ class RWBit:
     :param int register_width: The number of bytes in the register. Defaults to 1.
     """
     def __init__(self, register_address, bit, register_width=1):
-        self.bit_mask = 1 << bit
+        self.bit_mask = 1 << (bit%8) # the bitmask *within* the byte!
         self.buffer = bytearray(1 + register_width)
         self.buffer[0] = register_address
-        self.byte = bit // 8 + 1
+        self.byte = bit // 8 + 1  # the byte number within the buffer
 
     def __get__(self, obj, objtype=None):
         with obj.i2c_device as i2c:
diff --git a/adafruit_register/i2c_bits.py b/adafruit_register/i2c_bits.py
index 5afecd4..e67ac0a 100644
--- a/adafruit_register/i2c_bits.py
+++ b/adafruit_register/i2c_bits.py
@@ -48,12 +48,13 @@ def __init__(self, num_bits, register_address, lowest_bit, register_width=1):
         self.bit_mask = 0
         for _ in range(num_bits):
             self.bit_mask = (self.bit_mask << 1) + 1
-        self.bit_mask = self.bit_mask << lowest_bit
-        if self.bit_mask >= (1 << 8):
-            raise ValueError()
+        self.bit_mask = self.bit_mask << (lowest_bit%8)
+        if self.bit_mask >= 0xFF:
+            raise ValueError("Cannot have more bits than fit in a single byte")
+
         self.buffer = bytearray(1 + register_width)
         self.buffer[0] = register_address
-        self.lowest_bit = lowest_bit
+        self.lowest_bit = lowest_bit % 8  # lowest bit within byte
         self.byte = lowest_bit // 8 + 1
 
     def __get__(self, obj, objtype=None):