diff --git a/libtiff/tif_dirwrite.c b/libtiff/tif_dirwrite.c
index 2967da58..8d6686ba 100644
--- a/libtiff/tif_dirwrite.c
+++ b/libtiff/tif_dirwrite.c
@@ -2111,7 +2111,10 @@ TIFFWriteDirectoryTagCheckedLong8(TIFF* tif, uint32* ndir, TIFFDirEntry* dir, ui
 {
 	uint64 m;
 	assert(sizeof(uint64)==8);
-	assert(tif->tif_flags&TIFF_BIGTIFF);
+	if( !(tif->tif_flags&TIFF_BIGTIFF) ) {
+		TIFFErrorExt(tif->tif_clientdata,"TIFFWriteDirectoryTagCheckedLong8","LONG8 not allowed for ClassicTIFF");
+		return(0);
+	}
 	m=value;
 	if (tif->tif_flags&TIFF_SWAB)
 		TIFFSwabLong8(&m);
@@ -2124,7 +2127,10 @@ TIFFWriteDirectoryTagCheckedLong8Array(TIFF* tif, uint32* ndir, TIFFDirEntry* di
 {
 	assert(count<0x20000000);
 	assert(sizeof(uint64)==8);
-	assert(tif->tif_flags&TIFF_BIGTIFF);
+	if( !(tif->tif_flags&TIFF_BIGTIFF) ) {
+		TIFFErrorExt(tif->tif_clientdata,"TIFFWriteDirectoryTagCheckedLong8","LONG8 not allowed for ClassicTIFF");
+		return(0);
+	}
 	if (tif->tif_flags&TIFF_SWAB)
 		TIFFSwabArrayOfLong8(value,count);
 	return(TIFFWriteDirectoryTagData(tif,ndir,dir,tag,TIFF_LONG8,count,count*8,value));
@@ -2136,7 +2142,10 @@ TIFFWriteDirectoryTagCheckedSlong8(TIFF* tif, uint32* ndir, TIFFDirEntry* dir, u
 {
 	int64 m;
 	assert(sizeof(int64)==8);
-	assert(tif->tif_flags&TIFF_BIGTIFF);
+	if( !(tif->tif_flags&TIFF_BIGTIFF) ) {
+		TIFFErrorExt(tif->tif_clientdata,"TIFFWriteDirectoryTagCheckedLong8","SLONG8 not allowed for ClassicTIFF");
+		return(0);
+	}
 	m=value;
 	if (tif->tif_flags&TIFF_SWAB)
 		TIFFSwabLong8((uint64*)(&m));
@@ -2149,7 +2158,10 @@ TIFFWriteDirectoryTagCheckedSlong8Array(TIFF* tif, uint32* ndir, TIFFDirEntry* d
 {
 	assert(count<0x20000000);
 	assert(sizeof(int64)==8);
-	assert(tif->tif_flags&TIFF_BIGTIFF);
+	if( !(tif->tif_flags&TIFF_BIGTIFF) ) {
+		TIFFErrorExt(tif->tif_clientdata,"TIFFWriteDirectoryTagCheckedLong8","SLONG8 not allowed for ClassicTIFF");
+		return(0);
+	}
 	if (tif->tif_flags&TIFF_SWAB)
 		TIFFSwabArrayOfLong8((uint64*)value,count);
 	return(TIFFWriteDirectoryTagData(tif,ndir,dir,tag,TIFF_SLONG8,count,count*8,value));