diff --git a/src/portal.cpp b/src/portal.cpp
index f354faa65..4ba334225 100644
--- a/src/portal.cpp
+++ b/src/portal.cpp
@@ -225,7 +225,7 @@ static line_t *FindDestination(line_t *src, int tag)
 			}
 		}
 	}
-	return NULL;
+	return nullptr;
 }
 
 
@@ -275,7 +275,7 @@ static void SetRotation(FLinePortal *port)
 void P_SpawnLinePortal(line_t* line)
 {
 	// portal destination is special argument #0
-	line_t* dst = NULL;
+	line_t* dst = nullptr;
 
 	if (line->args[2] >= PORTT_VISUAL && line->args[2] <= PORTT_LINKED)
 	{
@@ -304,7 +304,7 @@ void P_SpawnLinePortal(line_t* line)
 				port->mType = PORTT_TELEPORT;
 			}
 		}
-		if (port->mDestination != NULL)
+		if (port->mDestination != nullptr)
 		{
 			port->mDefFlags = port->mType == PORTT_VISUAL ? PORTF_VISIBLE : port->mType == PORTT_TELEPORT ? PORTF_TYPETELEPORT : PORTF_TYPEINTERACTIVE;
 		}
@@ -357,7 +357,7 @@ void P_SpawnLinePortal(line_t* line)
 
 void P_UpdatePortal(FLinePortal *port)
 {
-	if (port->mDestination == NULL)
+	if (port->mDestination == nullptr)
 	{
 		// Portal has no destination: switch it off
 		port->mFlags = 0;
@@ -442,16 +442,16 @@ static bool ChangePortalLine(line_t *line, int destid)
 	if (line->portalindex >= linePortals.Size()) return false;
 	FLinePortal *port = &linePortals[line->portalindex];
 	if (port->mType == PORTT_LINKED) return false;	// linked portals cannot be changed.
-	if (destid == 0) port->mDestination = NULL;
+	if (destid == 0) port->mDestination = nullptr;
 	port->mDestination = FindDestination(line, destid);
-	if (port->mDestination == NULL)
+	if (port->mDestination == nullptr)
 	{
 		port->mFlags = 0;
 	}
 	else if (port->mType == PORTT_INTERACTIVE)
 	{
-		FLinePortal *portd = &linePortals[port->mDestination->portalindex];
-		if (portd != NULL && portd->mType == PORTT_INTERACTIVE && portd->mDestination == line)
+		FLinePortal *portd = port->mDestination->portalindex < linePortals.Size()? &linePortals[port->mDestination->portalindex] : nullptr;
+		if (portd != nullptr && portd->mType == PORTT_INTERACTIVE && portd->mDestination == line)
 		{
 			// this is a 2-way interactive portal
 			port->mFlags = port->mDefFlags | PORTF_INTERACTIVE;
@@ -662,7 +662,7 @@ void P_TranslatePortalZ(line_t* src, double& z)
 
 unsigned P_GetSkyboxPortal(AActor *actor)
 {
-	if (actor == NULL) return 1;	// this means a regular sky.
+	if (actor == nullptr) return 1;	// this means a regular sky.
 	for (unsigned i = 0;i<level.sectorPortals.Size();i++)
 	{
 		if (level.sectorPortals[i].mSkybox == actor) return i;
@@ -820,7 +820,7 @@ static bool CollectSectors(int groupid, sector_t *origin)
 		for (auto line : sec->Lines)
 		{
 			sector_t *other = line->frontsector == sec ? line->backsector : line->frontsector;
-			if (other != NULL && other != sec && other->PortalGroup != groupid)
+			if (other != nullptr && other != sec && other->PortalGroup != groupid)
 			{
 				other->PortalGroup = groupid;
 				list.Push(other);
@@ -1098,10 +1098,10 @@ void P_CreateLinkedPortals()
 	}
 
 	// reject would just get in the way when checking sight through portals.
-	if (Displacements.size > 1 && rejectmatrix != NULL)
+	if (Displacements.size > 1 && rejectmatrix != nullptr)
 	{
 		delete[] rejectmatrix;
-		rejectmatrix = NULL;
+		rejectmatrix = nullptr;
 	}
 	// finally we must flag all planes which are obstructed by the sector's own ceiling or floor.
 	for (auto &sec : level.sectors)