diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index 764191e..de63e3e 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -226,7 +226,7 @@ public:
 		toMont();
 	}
 	/*
-		mask inBuf with (1 << (bitLen - 1)) - 1
+		mask inBuf with (1 << (bitLen - 1)) - 1 if x >= p
 	*/
 	template<class S>
 	void setArrayMask(const S *inBuf, size_t n)
diff --git a/src/fp.cpp b/src/fp.cpp
index 7de679c..65fdc85 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -607,7 +607,10 @@ void copyAndMask(Unit *y, const void *x, size_t xByteSize, const Op& op, bool do
 		if (isGreaterOrEqualArray(y, op.p, op.N)) throw cybozu::Exception("fp:copyAndMask:large x");
 		return;
 	}
-	maskArray(y, op.N, op.bitSize - 1);
+	maskArray(y, op.N, op.bitSize);
+	if (isGreaterOrEqualArray(y, op.p, op.N)) {
+		maskArray(y, op.N, op.bitSize - 1);
+	}
 	assert(isLessArray(y, op.p, op.N));
 }
 
diff --git a/test/fp_test.cpp b/test/fp_test.cpp
index a601517..5c6a05c 100644
--- a/test/fp_test.cpp
+++ b/test/fp_test.cpp
@@ -545,7 +545,7 @@ void setArrayMaskTest2(mcl::fp::Mode mode)
 	} tbl[] = {
 		{ { 0x234567a4, 0x00000001, 0x00100000}, 1, "0x234567a4" },
 		{ { 0x234567a4, 0x00000001, 0x00100000}, 2, "0x1234567a4" },
-		{ { 0x234567a4, 0x00000001, 0x00100000}, 3, "0x00000000000001234567a4" },
+		{ { 0x234567a4, 0x00000001, 0x00100000}, 3, "0x10000000000001234567a4" },
 		{ { 0x234567a5, 0xfffffff1, 0xffffffff}, 3, "0x0ffffffffffff1234567a5" },
 	};
 	Fp x;