diff --git a/src/fp.cpp b/src/fp.cpp
index 4273e3f..c820937 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -156,7 +156,14 @@ void expand_message_xmd(uint8_t out[], size_t outSize, const void *msg, size_t m
 	const size_t r_in_bytes = 64;
 	const size_t n = outSize / mdSize;
 	static const uint8_t Z_pad[r_in_bytes] = {};
-	assert(dstSize < 256);
+	uint8_t largeDst[mdSize];
+	if (dstSize > 255) {
+		cybozu::Sha256 h;
+		h.update("H2C-OVERSIZE-DST-", 17);
+		h.digest(largeDst, mdSize, dst, dstSize);
+		dst = largeDst;
+		dstSize = mdSize;
+	}
 	/*
 		Z_apd | msg | BE(outSize, 2) | BE(0, 1) | DST | BE(dstSize, 1)
 	*/
diff --git a/test/mapto_wb19_test.cpp b/test/mapto_wb19_test.cpp
index 3c78b93..b6d4d11 100644
--- a/test/mapto_wb19_test.cpp
+++ b/test/mapto_wb19_test.cpp
@@ -361,6 +361,61 @@ void testHashToFp2v7(const T& mapto)
 		}
 	}
 	{
+		const uint8_t largeMsg[] = {
+			0xda, 0x56, 0x6f, 0xc9, 0xaf, 0x58, 0x60, 0x1b, 0xf6, 0xe0, 0x98, 0x25, 0x1c, 0x14, 0x03, 0x8e,
+			0x7f, 0x1b, 0x7d, 0xfe, 0x8f, 0x1c, 0xf4, 0x18, 0xa5, 0x3b, 0x4a, 0x86, 0x35, 0x9f, 0x67, 0xeb,
+			0x99, 0x6e, 0x30, 0xea, 0x3e, 0xcb, 0x8b, 0x32, 0x8a, 0xe2, 0x61, 0x5e, 0xde, 0xd5, 0x90, 0x20,
+			0xa7, 0xcc, 0x08, 0x14, 0x45, 0xe7, 0xa7, 0xe6, 0x1e, 0x53, 0x57, 0x34, 0x9f, 0x37, 0xfe, 0xb0,
+			0x25, 0xb1, 0x80, 0xf7, 0x1d, 0xee, 0xc3, 0xb0, 0xe8, 0x0f, 0xae, 0x85, 0xef, 0x42, 0x2a, 0x19,
+			0x98, 0xa4, 0x18, 0x15, 0x45, 0x5d, 0x63, 0x19, 0x64, 0x97, 0xeb, 0xd3, 0x54, 0x75, 0x9d, 0xe0,
+			0x7c, 0x20, 0x58, 0xef, 0x43, 0xb7, 0x06, 0x9e, 0x13, 0x69, 0x8b, 0x99, 0x49, 0x57, 0xd1, 0x82,
+			0x54, 0xa8, 0xb8, 0xff, 0x93, 0x81, 0x2b, 0xbb, 0x73, 0x05, 0x0d, 0x57, 0x51, 0x63, 0x48, 0x7d,
+			0x9c, 0xb8, 0xba, 0xd1, 0xb5, 0x31, 0x51, 0xf5, 0x06, 0xee, 0xf4, 0x96, 0xed, 0x18, 0x82, 0x53,
+			0xd8, 0xd4, 0xe0, 0xda, 0x2e, 0x4d, 0xfc, 0xc7, 0x4b, 0xa0, 0xba, 0xca, 0xa0, 0xf8, 0xfa, 0x86,
+			0x8a, 0x7e, 0xac, 0x9b, 0x76, 0x52, 0xab, 0xb8, 0xc5, 0x9d, 0xe3, 0x7e, 0xe1, 0x83, 0x3a, 0x92,
+			0x2c, 0x30, 0x96, 0x9a, 0x0e, 0xc3, 0xdc, 0x3f, 0xf2, 0x63, 0xf0, 0x27, 0x33, 0x3b, 0xbc, 0xf7,
+			0x40, 0x6d, 0x26, 0x54, 0x7e, 0x1e, 0x0f, 0xe6, 0x50, 0x75, 0x61, 0x4d, 0x1b, 0x9a, 0x02, 0x37,
+			0x49, 0xb3, 0xd4, 0x49, 0x3e, 0xe4, 0xc4, 0x23, 0x61, 0x53, 0xb4, 0x70, 0x17, 0x25, 0x88, 0xd3,
+			0xc4, 0x88, 0x2b, 0xf6, 0xd0, 0x96, 0x7d, 0x7e, 0xa3, 0x7c, 0x68, 0x0c, 0xa3, 0x5d, 0xd1, 0x4a,
+			0x31, 0x62, 0xa0, 0xe2, 0xb6, 0xb0, 0xb9, 0x70, 0x9b, 0x70, 0x00, 0xa5, 0x42, 0xbe, 0x5f, 0x1c,
+			0x12, 0xcd, 0xb9, 0x84, 0x6f, 0xb6, 0xf5, 0x80, 0xc4, 0xa9, 0xfa, 0xb6, 0x75, 0xca, 0xad, 0xc9,
+			0xe3, 0x3d, 0xf6, 0x63, 0x7a, 0x2a, 0xb7, 0x2a, 0xa0, 0xb4, 0xd9, 0xc2, 0xb9, 0x01, 0x3f, 0xd3,
+			0x2a, 0x3a, 0xd4, 0xfe, 0x58, 0x84, 0x7a, 0xee, 0xb0, 0x07, 0x18, 0x4a, 0x90, 0xe4, 0x96, 0xb3,
+			0x63, 0x43, 0xd5, 0xd4, 0x89, 0x4a, 0xc1, 0x4d, 0x6f, 0x25, 0x3d, 0xcc, 0x7b, 0xef, 0x2d, 0xef,
+			0x0a, 0xd7, 0x7a, 0x60, 0x8e, 0xfe, 0x0c, 0xc8, 0x66, 0x8c, 0xc3, 0xc7, 0xf7, 0xa9, 0x8a, 0x05,
+			0xab, 0x74, 0x43, 0x2d, 0xe6, 0x1a, 0xd6, 0xdb, 0x0c, 0xc0, 0x2a, 0xc0, 0x88, 0x88, 0x24, 0x7a,
+			0xbc, 0x9f, 0xae, 0xaf, 0x0c, 0x23, 0xa6, 0x0b, 0xe6, 0x3e, 0xf7, 0x32, 0xa8, 0x17, 0x87, 0xc4,
+			0xbc, 0xd1, 0x38, 0x6f, 0x8a, 0x94, 0xf7, 0xd4, 0x73, 0x85, 0xa5, 0x9e, 0xde, 0xce, 0x28, 0x6c,
+			0x32, 0x8e, 0x68, 0xeb, 0xda, 0xf1, 0x49, 0xbb, 0x34, 0x18, 0xb3, 0x84, 0xa8, 0x2e
+		};
+		const uint8_t largeDst[] = {
+			0xf0, 0x9d, 0x58, 0xbb, 0x9f, 0x78, 0xb7, 0x22, 0x3d, 0xa2, 0x77, 0xa8, 0x67, 0x83, 0xbb, 0x37,
+			0x52, 0x35, 0x1d, 0x08, 0x89, 0x43, 0x1d, 0x93, 0xf4, 0x00, 0xb3, 0x5c, 0x08, 0xdd, 0x7e, 0x68,
+			0x53, 0x90, 0x70, 0x9d, 0x79, 0x80, 0xb2, 0xe2, 0x21, 0x17, 0x6b, 0x20, 0xf0, 0xb2, 0xba, 0x39,
+			0x8c, 0xc1, 0xce, 0x12, 0x9c, 0xa0, 0xc6, 0x96, 0xc9, 0xc0, 0xce, 0xa4, 0xb2, 0x7b, 0x07, 0x29,
+			0x01, 0x41, 0x97, 0x09, 0x60, 0x76, 0xa9, 0x29, 0x6c, 0x7b, 0x5c, 0x6d, 0xd0, 0xb8, 0xdb, 0xc2,
+			0x32, 0x99, 0x49, 0x05, 0x27, 0x96, 0x73, 0x43, 0x27, 0x41, 0x0a, 0xb4, 0x90, 0x62, 0xd8, 0xcd,
+			0xc1, 0x20, 0xc7, 0x3a, 0x20, 0xa0, 0xb7, 0x99, 0x1d, 0x0a, 0x90, 0x93, 0x10, 0x84, 0x3f, 0xd2,
+			0x6c, 0x2d, 0x9e, 0xf8, 0x8f, 0x37, 0x9f, 0xab, 0xe1, 0xbf, 0x1f, 0x91, 0xc8, 0xee, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xbf, 0xce, 0x9d, 0x8f, 0x67, 0xa6,
+			0xda, 0x19, 0x4e, 0xa8, 0xe6, 0xc1, 0x5a, 0x7e, 0x25, 0xb0, 0x6c, 0xd0, 0x01, 0x0d, 0x6a, 0x44,
+			0xdc, 0xfd, 0x3d, 0x7a, 0x21, 0xa3, 0xce, 0x1a, 0x32, 0x2c, 0xfa, 0xce, 0x75, 0xb5, 0xaf, 0x3d,
+			0x55, 0x6d, 0xd0, 0x06, 0x32, 0x72, 0x43, 0xd2, 0x72, 0xf0, 0xed, 0x00, 0x00, 0x00
+		};
+
 		const struct {
 			const char *msg;
 			const char *dst;
@@ -478,12 +533,28 @@ void testHashToFp2v7(const T& mapto)
 					"0x03a47f8e6d1763ba0cad63d6114c0accbef65707825a511b251a660a9b3994249ae4e63fac38b23da0c398689ee2ab52",
 				}
 			},
+			{
+				(const char*)largeMsg,
+				(const char*)largeDst,
+				{
+					"1621198997308164855924679271466068736858072552785749054645341480120465596875824494080137207843099766374306872449248",
+					"1083281626011882732241684404246060395112782337887047566845349523874158040278646833293564936724890128537465803996399",
+				},
+				{
+					"3796472889358084161418545238718592151842923587110768242624300389546030860679313089401746511331063864568256792368175",
+					"3428585241079663876306492378538920664201248193908204300130152438040549594213769385563811847347660927609220812378737",
+				}
+			},
 		};
 		for (size_t i = 0; i < CYBOZU_NUM_OF_ARRAY(tbl); i++) {
 			const char *msg = tbl[i].msg;
 			size_t msgSize = strlen(msg);
 			const char *dst = tbl[i].dst;
 			size_t dstSize = strlen(dst);
+			if (msg == (const char*)largeMsg) {
+				msgSize = sizeof(largeMsg);
+				dstSize = sizeof(largeDst);
+			}
 			G2 P1, P2;
 			set(P1.x, tbl[i].x);
 			set(P1.y, tbl[i].y);