diff --git a/pyModeS/decoder/allcall.py b/pyModeS/decoder/allcall.py
index 5f352f4..05ef31c 100644
--- a/pyModeS/decoder/allcall.py
+++ b/pyModeS/decoder/allcall.py
@@ -42,8 +42,15 @@ def interrogator(msg):
         int: interrogator identifier code
 
     """
-    # simply the CRC reminder
-    return common.crc(msg)
+    # the CRC remainder contains the CL and IC field. top three bits are CL field and last four bits are IC field.
+    remainder = common.crc(msg)
+    if remainder > 79: 
+        IC = "corrupt IC"
+    elif remainder < 16:
+        IC="II"+str(remainder)
+    else:
+        IC="SI"+str(remainder-16)
+    return IC
 
 
 @_checkdf