diff --git a/tests/uchar_test.cpp b/tests/uchar_test.cpp
index c887f8a..8b29667 100644
--- a/tests/uchar_test.cpp
+++ b/tests/uchar_test.cpp
@@ -280,7 +280,10 @@
 
   char bytes[MB_LEN_MAX];
 
+  memset(bytes, 1, sizeof(bytes));
   EXPECT_EQ(1U, c32rtomb(bytes, L'\0', NULL));
+  EXPECT_EQ('\0', bytes[0]);
+  EXPECT_EQ('\x01', bytes[1]);
 
   memset(bytes, 0, sizeof(bytes));
   EXPECT_EQ(1U, c32rtomb(bytes, L'h', NULL));
@@ -408,4 +411,3 @@
   GTEST_LOG_(INFO) << "uchar.h is unavailable.\n";
 #endif
 }
-
diff --git a/tests/wchar_test.cpp b/tests/wchar_test.cpp
index 830eb70..097647f 100644
--- a/tests/wchar_test.cpp
+++ b/tests/wchar_test.cpp
@@ -445,6 +445,18 @@
   ASSERT_EQ(L'e', dst[1]);
   ASSERT_EQ(L'l', dst[2]);
   ASSERT_EQ(&s[3], src);
+
+  memset(dst, 0, sizeof(dst));
+  const char* incomplete = "\xc2"; // Incomplete UTF-8 sequence.
+  src = incomplete;
+  errno = 0;
+  ASSERT_EQ(static_cast<size_t>(-1), mbsnrtowcs(dst, &src, SIZE_MAX, 3, nullptr));
+  ASSERT_EQ(EILSEQ, errno);
+
+  src = incomplete;
+  errno = 0;
+  ASSERT_EQ(static_cast<size_t>(-1), mbsnrtowcs(nullptr, &src, SIZE_MAX, 3, nullptr));
+  ASSERT_EQ(EILSEQ, errno);
 }
 
 TEST(wchar, wcsftime) {