diff --git a/src/d_main.cpp b/src/d_main.cpp
index 41040b753..9b6b9d5db 100644
--- a/src/d_main.cpp
+++ b/src/d_main.cpp
@@ -1852,6 +1852,10 @@ void D_SRB2Main(void)
 				I_Error("Bad '%s' level warp.\n"
 #if defined (_WIN32)
 				"Are you using MSDOS 8.3 filenames in Zone Builder?\n"
+				"\n"
+				"To check: edit the BlanKart game configuration in Ultimate Lowee Builder.\n"
+				"Go to the Testing tab and make sure \"Use short paths and file names\" is turned off.\n"
+				"(The option is hidden by default. Check \"Customize parameters\" to show it.)\n"
 #endif
 				, word);
 		}
diff --git a/src/p_setup.c b/src/p_setup.c
index 60a1e4db9..0acc4990b 100644
--- a/src/p_setup.c
+++ b/src/p_setup.c
@@ -9269,7 +9269,7 @@ INT16 wadnamemap = 0; // gamemap based
 UINT8 P_InitMapData(boolean existingmapheaders)
 {
 	UINT8 ret = 0;
-	INT32 i;
+	INT32 i, j;
 	lumpnum_t maplump;
 	virtres_t *virtmap;
 	virtlump_t *minimap, *thumbnailPic;
@@ -9279,27 +9279,13 @@ UINT8 P_InitMapData(boolean existingmapheaders)
 	for (i = 0; i < nummapheaders; ++i)
 	{
 		name = mapheaderinfo[i]->lumpname;
-		maplump = W_CheckNumForMap(name);
-
-		// Doesn't exist?
-		if (maplump == INT16_MAX)
-		{
-#ifndef DEVELOP
-			if (!existingmapheaders)
-			{
-				I_Error("P_InitMapData: Base map %s has a header but no level\n", name);
-			}
-#endif
-			continue;
-		}
+		maplump = W_CheckNumForMap(name, (mapheaderinfo[i]->lumpnum == LUMPERROR));
 
 		// Always check for cup cache reassociations.
 		// (The core assumption is that cups < headers.)
+		if (maplump != LUMPERROR || mapheaderinfo[i]->lumpnum != LUMPERROR)
 		{
 			cupheader_t *cup = kartcupheaders;
-			INT32 j;
-
-			mapheaderinfo[i]->cup = NULL;
 
 			while (cup)
 			{
@@ -9327,6 +9313,18 @@ UINT8 P_InitMapData(boolean existingmapheaders)
 			}
 		}
 
+		// Doesn't exist in this set of files?
+		if (maplump == LUMPERROR)
+		{
+#ifndef DEVELOP
+			if (!existingmapheaders)
+			{
+				I_Error("P_InitMapData: Base map %s has a header but no level\n", name);
+			}
+#endif
+			continue;
+		}
+
 		// No change?
 		if (mapheaderinfo[i]->lumpnum == maplump)
 			continue;
@@ -9350,6 +9348,7 @@ UINT8 P_InitMapData(boolean existingmapheaders)
 			}
 
 			mapheaderinfo[i]->lumpnum = maplump;
+
 			if (maplump == wadnamelump)
 				wadnamemap = i+1;
 
diff --git a/src/w_wad.c b/src/w_wad.c
index 3b091c465..5fde6a0df 100644
--- a/src/w_wad.c
+++ b/src/w_wad.c
@@ -1452,11 +1452,12 @@ lumpnum_t W_CheckNumForLongName(const char *name)
 
 // Look for valid map data through all added files in descendant order.
 // Get a map marker for WADs, and a standalone WAD file lump inside PK3s.
-lumpnum_t W_CheckNumForMap(const char *name)
+lumpnum_t W_CheckNumForMap(const char *name, boolean checktofirst)
 {
 	lumpnum_t check = INT16_MAX;
 	UINT32 uhash, hash = quickncasehash(name, LUMPNUMCACHENAME);
 	INT32 i;
+	UINT16 firstfile = (checktofirst || (partadd_earliestfile == UINT16_MAX)) ? 0 : partadd_earliestfile;
 
 	// Check the lumpnumcache first. Loop backwards so that we check
 	// most recent entries first
@@ -1472,7 +1473,7 @@ lumpnum_t W_CheckNumForMap(const char *name)
 
 	uhash = quickncasehash(name, 8); // Not a mistake, legacy system for short lumpnames
 
-	for (i = numwadfiles - 1; i >= 0; i--)
+	for (i = numwadfiles - 1; i >= firstfile; i--)
 	{
 		check = W_CheckNumForMapPwad(name, uhash, (UINT16)i, 0);
 
diff --git a/src/w_wad.h b/src/w_wad.h
index a663c8d59..d64a90782 100644
--- a/src/w_wad.h
+++ b/src/w_wad.h
@@ -171,7 +171,7 @@ UINT16 W_CheckNumForFullNamePK3(const char *name, UINT16 wad, UINT16 startlump);
 UINT16 W_CheckNumForFolderStartPK3(const char *name, UINT16 wad, UINT16 startlump);
 UINT16 W_CheckNumForFolderEndPK3(const char *name, UINT16 wad, UINT16 startlump);
 
-lumpnum_t W_CheckNumForMap(const char *name);
+lumpnum_t W_CheckNumForMap(const char *name, boolean checktofirst);
 lumpnum_t W_CheckNumForName(const char *name);
 lumpnum_t W_CheckNumForLongName(const char *name);
 lumpnum_t W_GetNumForName(const char *name); // like W_CheckNumForName but I_Error on LUMPERROR