diff --git a/src/common/DynamicStrings.cpp b/src/common/DynamicStrings.cpp
index 72be2e07ae6..1e02df3e874 100644
--- a/src/common/DynamicStrings.cpp
+++ b/src/common/DynamicStrings.cpp
@@ -87,6 +87,8 @@ unsigned makeDynamicStrings(unsigned length, ISC_STATUS* const dst, const ISC_ST
 		case isc_arg_cstring:
 			fb_assert(string);
 			*to++ = (ISC_STATUS)(IPTR) string;
+			if (!string)
+				break;
 			memcpy(string, reinterpret_cast<const char*>(from[1]), from[0]);
 			string += *from++;
 			*string++ = '\0';
@@ -97,6 +99,8 @@ unsigned makeDynamicStrings(unsigned length, ISC_STATUS* const dst, const ISC_ST
 		case isc_arg_sql_state:
 			fb_assert(string);
 			*to++ = (ISC_STATUS)(IPTR) string;
+			if (!string)
+				break;
 			strcpy(string, reinterpret_cast<const char*>(*from));
 			string += strlen(string);
 			string++;
diff --git a/src/gpre/obj_cxx.cpp b/src/gpre/obj_cxx.cpp
index b863f2cf3e6..aae53bdc94f 100644
--- a/src/gpre/obj_cxx.cpp
+++ b/src/gpre/obj_cxx.cpp
@@ -3368,7 +3368,8 @@ static void gen_t_start( const act* action, int column)
 
 	if (trans->tra_db_count == 1)
 	{
-		printa(column, "%s = %s->startTransaction(%s, %d, fb_tpb_%d);",
+		if (trans->tra_tpb) 
+			printa(column, "%s = %s->startTransaction(%s, %d, fb_tpb_%d);",
 			trans->tra_handle ? trans->tra_handle : gpreGlob.transaction_name,
 			trans->tra_tpb->tpb_database->dbb_name->sym_string, vector,
 			trans->tra_tpb->tpb_length, trans->tra_tpb->tpb_ident);
diff --git a/src/jrd/lck.cpp b/src/jrd/lck.cpp
index 8a3baf6bf8d..89f30736a78 100644
--- a/src/jrd/lck.cpp
+++ b/src/jrd/lck.cpp
@@ -1049,6 +1049,9 @@ static Lock* hash_get_lock(Lock* lock, USHORT* hash_slot, Lock*** prior)
 
 	// if no collisions found, we're done
 
+	if (!att->att_compatibility_table)
+		return NULL;
+
 	Lock* match = (*att->att_compatibility_table)[hash_value];
 	if (!match)
 		return NULL;
@@ -1333,6 +1336,9 @@ static USHORT internal_downgrade(thread_db* tdbb, CheckStatusWrapper* statusVect
 
 	// if we can convert to that level, set all identical locks as having that level
 
+	if (!first || !first->lck_physical)
+		return NULL;
+	
 	if (level < first->lck_physical)
 	{
 		if (dbb->lockManager()->convert(tdbb, statusVector, first->lck_id, level, LCK_NO_WAIT,
diff --git a/src/jrd/replication/Applier.cpp b/src/jrd/replication/Applier.cpp
index c9e9f39419a..88a3a1d09e1 100644
--- a/src/jrd/replication/Applier.cpp
+++ b/src/jrd/replication/Applier.cpp
@@ -998,7 +998,7 @@ void Applier::storeBlob(thread_db* tdbb, TraNumber traNum, bid* blobId,
 	fb_assert(blob->blb_flags & BLB_temporary);
 	fb_assert(!(blob->blb_flags & BLB_closed));
 
-	if (length)
+	if (blob && length)
 		blob->BLB_put_segment(tdbb, data, length);
 	else
 		blob->BLB_close(tdbb);