diff --git a/gsmmodem/pdu.py b/gsmmodem/pdu.py
index a3e28e8..0ec07e4 100644
--- a/gsmmodem/pdu.py
+++ b/gsmmodem/pdu.py
@@ -465,8 +465,11 @@ def _decodeUserData(byteIter, userDataLen, dataCoding, udhPresent):
             # Since we are using 7-bit data, "fill bits" may have been added to make the UDH end on a septet boundary
             shift = ((udhLen + 1) * 8) % 7 # "fill bits" needed to make the UDH end on a septet boundary
             # Simulate another "shift" in the unpackSeptets algorithm in order to ignore the fill bits
-            prevOctet = next(byteIter)
-            shift += 1
+            if shift:
+                prevOctet = next(byteIter)
+                shift += 1
+            else:
+                prevOctet = 0
 
     if dataCoding == 0x00: # GSM-7
         if udhPresent: