diff --git a/README.md b/README.md
index ef9a38e..217b139 100644
--- a/README.md
+++ b/README.md
@@ -168,8 +168,7 @@ is how you can run them:
 ```sh
 mkdir build && cd build
 cmake ..
-make python_install
-make check
+make check-ut
 ```
 
 To add new tests for the feature you are working one, see the other tests in
diff --git a/include/osdp.h b/include/osdp.h
index 3651b44..9992ee9 100644
--- a/include/osdp.h
+++ b/include/osdp.h
@@ -72,6 +72,17 @@ extern "C" {
  */
 #define OSDP_FLAG_CAPTURE_PACKETS 0x00100000
 
+/**
+ * @brief Allow an empty encrypted data block(SCS_17 and SCS_18 packets).
+ * This is non-conforming to the standard.  If there is no data to be
+ * transferred, the CP should instead use the SCS_15/SCS_16 messages.
+ * Some OSDP implementations are buggy and send a 0-length data block with
+ * the SCS_17 and SCS_18 messages, this flag accepts that buggy behavior.
+ *
+ * @note this is a PD mode only flag
+ */
+#define OSDP_FLAG_ALLOW_EMPTY_ENCRYPTED_DATA_BLOCK 0x00200000
+
 /**
  * @brief Various PD capability function codes.
  */
diff --git a/python/osdp/constants.py b/python/osdp/constants.py
index 4d0c583..2e38df2 100644
--- a/python/osdp/constants.py
+++ b/python/osdp/constants.py
@@ -12,6 +12,7 @@ class LibFlag:
     IgnoreUnsolicited = osdp_sys.FLAG_IGN_UNSOLICITED
     EnableNotification = osdp_sys.FLAG_ENABLE_NOTIFICATION
     CapturePackets = osdp_sys.FLAG_CAPTURE_PACKETS
+    AllowEmptyEncryptedDataBlock = osdp_sys.FLAG_ALLOW_EMPTY_ENCRYPTED_DATA_BLOCK
 
 class LogLevel:
     Emergency = osdp_sys.LOG_EMERG
diff --git a/python/osdp_sys/module.c b/python/osdp_sys/module.c
index 7d935ed..b5d365e 100644
--- a/python/osdp_sys/module.c
+++ b/python/osdp_sys/module.c
@@ -24,6 +24,7 @@ void pyosdp_add_module_constants(PyObject *module)
 	ADD_CONST("FLAG_IGN_UNSOLICITED", OSDP_FLAG_IGN_UNSOLICITED);
 	ADD_CONST("FLAG_ENABLE_NOTIFICATION", OSDP_FLAG_ENABLE_NOTIFICATION);
 	ADD_CONST("FLAG_CAPTURE_PACKETS", OSDP_FLAG_CAPTURE_PACKETS);
+	ADD_CONST("FLAG_ALLOW_EMPTY_ENCRYPTED_DATA_BLOCK", OSDP_FLAG_ALLOW_EMPTY_ENCRYPTED_DATA_BLOCK);
 
 	ADD_CONST("LOG_EMERG", OSDP_LOG_EMERG);
 	ADD_CONST("LOG_ALERT", OSDP_LOG_ALERT);
diff --git a/src/osdp_common.h b/src/osdp_common.h
index 2bd43cf..f296f5f 100644
--- a/src/osdp_common.h
+++ b/src/osdp_common.h
@@ -43,6 +43,14 @@
 #define LOG_ERR(...)   __logger_log(&pd->logger, LOG_ERR,    __FILE__, __LINE__, __VA_ARGS__)
 #define LOG_INF(...)   __logger_log(&pd->logger, LOG_INFO,   __FILE__, __LINE__, __VA_ARGS__)
 #define LOG_WRN(...)   __logger_log(&pd->logger, LOG_WARNING,__FILE__, __LINE__, __VA_ARGS__)
+#define LOG_WRN_ONCE(...) \
+do {\
+  static int warned = 0; \
+  if(!warned) { \
+    __logger_log(&pd->logger, LOG_WARNING,__FILE__, __LINE__, __VA_ARGS__);\
+    warned = 1;\
+  }\
+}while(0)
 #define LOG_NOT(...)   __logger_log(&pd->logger, LOG_NOTICE, __FILE__, __LINE__, __VA_ARGS__)
 #define LOG_DBG(...)   __logger_log(&pd->logger, LOG_DEBUG,  __FILE__, __LINE__, __VA_ARGS__)
 
@@ -581,4 +589,8 @@ static inline bool is_packet_trace_enabled(struct osdp_pd *pd) {
 	        IS_ENABLED(CONFIG_OSDP_PACKET_TRACE));
 }
 
+static inline bool sc_allow_empty_encrypted_data_block(struct osdp_pd *pd) {
+	return ISSET_FLAG(pd, OSDP_FLAG_ALLOW_EMPTY_ENCRYPTED_DATA_BLOCK);
+}
+
 #endif	/* _OSDP_COMMON_H_ */
diff --git a/src/osdp_phy.c b/src/osdp_phy.c
index 2274500..1d6bc04 100644
--- a/src/osdp_phy.c
+++ b/src/osdp_phy.c
@@ -664,7 +664,7 @@ int osdp_phy_decode_packet(struct osdp_pd *pd, uint8_t **pkt_start)
 				 * used SCS_15/SCS_16 but we will be tolerant
 				 * towards those faulty implementations.
 				 */
-				LOG_INF("Received encrypted data block with 0 "
+				LOG_WRN_ONCE("Received encrypted data block with 0 "
 					"length; tolerating non-conformance!");
 			}
 			len += 1; /* put back cmd/reply ID */
diff --git a/src/osdp_sc.c b/src/osdp_sc.c
index 35b448d..a695157 100644
--- a/src/osdp_sc.c
+++ b/src/osdp_sc.c
@@ -139,6 +139,10 @@ int osdp_decrypt_data(struct osdp_pd *pd, int is_cmd, uint8_t *data, int length)
 		return -1;
 	}
 
+	if (sc_allow_empty_encrypted_data_block(pd) && length == 0) {
+		return 0;
+	}
+
 	memcpy(iv, is_cmd ? pd->sc.r_mac : pd->sc.c_mac, 16);
 	for (i = 0; i < 16; i++) {
 		iv[i] = ~iv[i];