diff --git a/tests/mock_smbus2.py b/tests/mock_smbus2.py
index dcfac70..bdd8d20 100644
--- a/tests/mock_smbus2.py
+++ b/tests/mock_smbus2.py
@@ -10,16 +10,17 @@ answers = {
     (0x20, 0x15): add_crc([0,0]) ,#GET_FEATURES
     (0x20, 0x03): None ,#IAQ_INIT
     (0x20, 0x08): [1, 144, 76, 0, 6, 39] ,#IAQ_MEASURE
-    (0x20, 0x32): add_crc([0xD4,0x00]),#IAQ_SELFTEST
-    (0x20, 0x15): add_crc([0x00,0x00])*2 ,#GET_BASELINE  Invalid, should be more realistic data..
+    (0x20, 0x32): add_crc([0xD4,0x00]) ,#IAQ_SELFTEST
+    (0x20, 0x15): [133, 152, 85, 138, 32, 202] ,#GET_BASELINE
     (0x20, 0x1e): None,#SET_BASELINE
 }
 
 class MockSMBus:
-    def __init__(s):
+    def __init__(s,break_crc=False):
         s.status=None
         s.last=None
         s.addr=None
+        s._break_crc=break_crc
 
     def i2c_rdwr(s,*msgs):
         for m in msgs:
@@ -38,6 +39,10 @@ class MockSMBus:
             raise AssertionError("tired to read before write")
         for i in range(len(s.status)):
             msg.buf[i]=chr(s.status[i])
+            if s._break_crc and i%3 == 2:
+                msg.buf[i]=msg.buf[i]^42
+                
+
     def _process_write(s,msg):
         s.status = answers[tuple(msg)]
         s.last=msg