diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index 6ec11fa..d15176f 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -274,8 +274,8 @@ public:
 			fp::copyUnitToByteAsLE(reinterpret_cast<uint8_t*>(&str[0]), p, str.size());
 			return;
 		}
-		// use low 5-bit ioMode for base
-		fp::arrayToStr(str, b.p, b.n, ioMode & 31);
+		// use low 8-bit ioMode for Fp
+		fp::arrayToStr(str, b.p, b.n, ioMode & 255);
 	}
 	std::string getStr(int ioMode = 0) const
 	{
diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index b91bedc..c660fc6 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -72,18 +72,18 @@ namespace mcl {
 */
 enum IoMode {
 	IoAuto = 0, // dec or hex according to ios_base::fmtflags
-	IoPrefix = 1, // append '0b'(bin) or '0x'(hex)
 	IoBin = 2, // binary number without prefix
 	IoDec = 10, // decimal number without prefix
 	IoHex = 16, // hexadecimal number without prefix
-	IoBinPrefix = IoBin | IoPrefix,
-	IoHexPrefix = IoHex | IoPrefix,
 	IoArray = 32, // array of Unit(fixed size)
 	IoArrayRaw = 64, // raw array of Unit without Montgomery conversion
+	IoPrefix = 128, // append '0b'(bin) or '0x'(hex)
+	IoBinPrefix = IoBin | IoPrefix,
+	IoHexPrefix = IoHex | IoPrefix,
 	IoEcAffine = 0, // affine coordinate
-	IoEcCompY = 128, // 1-bit y representation of elliptic curve
-	IoEcComp = 256, // use MBS for 1-bit y
-	IoEcProj = 512, // projective or jacobi coordinate
+	IoEcCompY = 256, // 1-bit y representation of elliptic curve
+	IoEcComp = 512, // use MBS for 1-bit y
+	IoEcProj = 1024, // projective or jacobi coordinate
 	IoTight = IoEcComp // tight repr of Ec(obsolete)
 };
 
diff --git a/src/fp.cpp b/src/fp.cpp
index c99a3fe..debebd9 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -538,8 +538,8 @@ void streamToArray(bool *pIsMinus, Unit *x, size_t byteSize, std::istream& is, i
 		is >> str;
 		const char *p = verifyStr(pIsMinus, &ioMode, str);
 		mpz_class mx;
-		// check low 5-bit of ioMode
-		if (!gmp::setStr(mx, p, ioMode & (31 & ~IoPrefix))) {
+		// use low 8-bit ioMode for Fp
+		if (!gmp::setStr(mx, p, ioMode & (255 & ~IoPrefix))) {
 			throw cybozu::Exception("fp:streamToArray:bad format") << ioMode << str;
 		}
 		const size_t n = (byteSize + sizeof(Unit) - 1) / sizeof(Unit);