diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 69f489627..e005c3c67 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -4760,13 +4760,13 @@ static void SetActorTeleFog(AActor *activator, int tid, FName telefogsrc, FName
 		if (activator != NULL)
 		{
 			check = PClass::FindClass(telefogsrc);
-			if (check != NULL || !stricmp(telefogsrc, "none") || !stricmp(telefogsrc, "null"))
+			if (check == NULL || !stricmp(telefogsrc, "none") || !stricmp(telefogsrc, "null"))
 				activator->TeleFogSourceType = NULL;
 			else
 				activator->TeleFogSourceType = check;
 
 			check = PClass::FindClass(telefogdest);
-			if (check != NULL || !stricmp(telefogdest, "none") || !stricmp(telefogdest, "null"))
+			if (check == NULL || !stricmp(telefogdest, "none") || !stricmp(telefogdest, "null"))
 				activator->TeleFogDestType = NULL;
 			else
 				activator->TeleFogDestType = check;
@@ -4780,13 +4780,13 @@ static void SetActorTeleFog(AActor *activator, int tid, FName telefogsrc, FName
 		while ((actor = iterator.Next()))
 		{
 			check = PClass::FindClass(telefogsrc);
-			if (check != NULL || !stricmp(telefogsrc, "none") || !stricmp(telefogsrc, "null"))
+			if (check == NULL || !stricmp(telefogsrc, "none") || !stricmp(telefogsrc, "null"))
 				actor->TeleFogSourceType = NULL;
 			else
 				actor->TeleFogSourceType = check;
 
 			check = PClass::FindClass(telefogdest);
-			if (check != NULL || !stricmp(telefogdest, "none") || !stricmp(telefogdest, "null"))
+			if (check == NULL || !stricmp(telefogdest, "none") || !stricmp(telefogdest, "null"))
 				actor->TeleFogDestType = NULL;
 			else
 				actor->TeleFogDestType = check;