diff --git a/bacnet-stack/iam.c b/bacnet-stack/iam.c
index b26a74ba..0ea832b5 100755
--- a/bacnet-stack/iam.c
+++ b/bacnet-stack/iam.c
@@ -80,50 +80,66 @@ int iam_decode_apdu(
   uint16_t *pVendor_id)
 {
   int len = 0;
+  int apdu_len = 0; // total length of the apdu, return value
   int object_type = 0; // should be a Device Object
   uint32_t object_instance = 0;
   uint8_t tag_number = 0;
   uint32_t len_value = 0;
   unsigned int decoded_value = 0;
 
+  // valid data?
+  if (!apdu)
+    return -1;
   // optional checking - most likely was already done prior to this call
   if (apdu[0] != PDU_TYPE_UNCONFIRMED_SERVICE_REQUEST)
     return -1;
   if (apdu[1] != SERVICE_UNCONFIRMED_I_AM)
     return -1;
   // OBJECT ID - object id
-  len = 2;
-  len += decode_tag_number_and_value(&apdu[len], &tag_number, &len_value);
+  apdu_len = 2;
+  len = decode_tag_number_and_value(&apdu[apdu_len], &tag_number, &len_value);
+  apdu_len += len;
   if (tag_number != BACNET_APPLICATION_TAG_OBJECT_ID)
     return -1;
-  len += decode_object_id(&apdu[len], &object_type, &object_instance);
+  len = decode_object_id(&apdu[apdu_len], &object_type, &object_instance);
+  apdu_len += len;
   if (object_type != OBJECT_DEVICE)
     return -1;
-  *pDevice_id = object_instance;
+  if (pDevice_id)
+    *pDevice_id = object_instance;
   // MAX APDU - unsigned
-  len += decode_tag_number_and_value(&apdu[len], &tag_number, &len_value);
+  len = decode_tag_number_and_value(&apdu[apdu_len], &tag_number, &len_value);
+  apdu_len += len;
   if (tag_number != BACNET_APPLICATION_TAG_UNSIGNED_INT)
     return -1;
-  len += decode_unsigned(&apdu[len], len_value, &decoded_value);
-  *pMax_apdu = decoded_value;
+  len = decode_unsigned(&apdu[apdu_len], len_value, &decoded_value);
+  apdu_len += len;
+  if (pMax_apdu)
+    *pMax_apdu = decoded_value;
   // Segmentation - enumerated
-  len += decode_tag_number_and_value(&apdu[len], &tag_number, &len_value);
+  len = decode_tag_number_and_value(&apdu[apdu_len], &tag_number, &len_value);
+  apdu_len += len;
   if (tag_number != BACNET_APPLICATION_TAG_ENUMERATED)
     return -1;
-  len += decode_enumerated(&apdu[len],len_value, &decoded_value);
+  len = decode_enumerated(&apdu[apdu_len],len_value, &decoded_value);
+  apdu_len += len;
   if (decoded_value >= MAX_BACNET_SEGMENTATION)
     return -1;
-  *pSegmentation = decoded_value;
+  if (pSegmentation)
+    *pSegmentation = decoded_value;
   // Vendor ID - unsigned16
-  len += decode_tag_number_and_value(&apdu[len], &tag_number, &len_value);
+  len = decode_tag_number_and_value(&apdu[apdu_len], &tag_number, &len_value);
+  apdu_len += len;
   if (tag_number != BACNET_APPLICATION_TAG_UNSIGNED_INT)
     return -1;
-  len += decode_unsigned(&apdu[len], len_value, &decoded_value);
+  len = decode_unsigned(&apdu[apdu_len], len_value, &decoded_value);
+  apdu_len += len;
   if (decoded_value > 0xFFFF)
     return -1;
-  *pVendor_id = decoded_value;
+  if (pVendor_id)
+    *pVendor_id = decoded_value;
   
-  return len;
+  return apdu_len;
 }
 
 #ifdef TEST
@@ -158,15 +174,12 @@ void testIAm(Test * pTest)
     &test_max_apdu,
     &test_segmentation,
     &test_vendor_id);
+    
   ct_test(pTest, len != -1);
   ct_test(pTest, test_device_id == device_id);
   ct_test(pTest, test_vendor_id == vendor_id);
   ct_test(pTest, test_max_apdu == max_apdu);
   ct_test(pTest, test_segmentation == segmentation);
-
-  // check the error conditions
-
-  
 }
 
 #ifdef TEST_IAM
@@ -187,5 +200,5 @@ int main(void)
 
     return 0;
 }
-#endif                          /* TEST_DECODE */
+#endif                          /* TEST_IAM */
 #endif                          /* TEST */