diff --git a/libc/bionic/wchar.cpp b/libc/bionic/wchar.cpp
index e0879b9..d28888d 100644
--- a/libc/bionic/wchar.cpp
+++ b/libc/bionic/wchar.cpp
@@ -70,21 +70,19 @@
   mbstate_t* state = (ps == NULL) ? &__private_state : ps;
   size_t i, o, r;
 
+  // The fast paths in the loops below are not safe if an ASCII
+  // character appears as anything but the first byte of a
+  // multibyte sequence. Check now to avoid doing it in the loops.
+  if (nmc > 0 && mbstate_bytes_so_far(state) > 0 && static_cast<uint8_t>((*src)[0]) < 0x80) {
+    return reset_and_return_illegal(EILSEQ, state);
+  }
+
+  // Measure only?
   if (dst == NULL) {
-    /*
-     * The fast path in the loop below is not safe if an ASCII
-     * character appears as anything but the first byte of a
-     * multibyte sequence. Check now to avoid doing it in the loop.
-     */
-    if ((nmc > 0) && (mbstate_bytes_so_far(state) > 0)
-        && (static_cast<uint8_t>((*src)[0]) < 0x80)) {
-      return reset_and_return_illegal(EILSEQ, state);
-    }
     for (i = o = 0; i < nmc; i += r, o++) {
       if (static_cast<uint8_t>((*src)[i]) < 0x80) {
         // Fast path for plain ASCII characters.
         if ((*src)[i] == '\0') {
-          *src = nullptr;
           return reset_and_return(o, state);
         }
         r = 1;
@@ -97,7 +95,6 @@
           return reset_and_return_illegal(EILSEQ, state);
         }
         if (r == 0) {
-          *src = nullptr;
           return reset_and_return(o, state);
         }
       }
@@ -105,15 +102,7 @@
     return reset_and_return(o, state);
   }
 
-  /*
-   * The fast path in the loop below is not safe if an ASCII
-   * character appears as anything but the first byte of a
-   * multibyte sequence. Check now to avoid doing it in the loop.
-   */
-  if ((nmc > 0) && (mbstate_bytes_so_far(state) > 0)
-      && (static_cast<uint8_t>((*src)[0]) < 0x80)) {
-    return reset_and_return_illegal(EILSEQ, state);
-  }
+  // Actually convert, updating `dst` and `src`.
   for (i = o = 0; i < nmc && o < len; i += r, o++) {
     if (static_cast<uint8_t>((*src)[i]) < 0x80) {
       // Fast path for plain ASCII characters.
@@ -151,7 +140,7 @@
   static mbstate_t __private_state;
   mbstate_t* state = (ps == NULL) ? &__private_state : ps;
 
-  // Our wchar_t is UTF-32
+  // Our wchar_t is UTF-32.
   return c32rtomb(s, static_cast<char32_t>(wc), state);
 }
 
diff --git a/tests/wchar_test.cpp b/tests/wchar_test.cpp
index db51c08..34ed5a7 100644
--- a/tests/wchar_test.cpp
+++ b/tests/wchar_test.cpp
@@ -303,7 +303,7 @@
   ASSERT_EQ(EILSEQ, errno);
 }
 
-void test_mbrtowc_incomplete(mbstate_t* ps) {
+static void test_mbrtowc_incomplete(mbstate_t* ps) {
   ASSERT_STREQ("C.UTF-8", setlocale(LC_CTYPE, "C.UTF-8"));
   uselocale(LC_GLOBAL_LOCALE);
 
@@ -340,10 +340,13 @@
   test_mbrtowc_incomplete(NULL);
 }
 
-void test_mbsrtowcs(mbstate_t* ps) {
+static void test_mbsrtowcs(mbstate_t* ps) {
+  constexpr const char* VALID = "A" "\xc2\xa2" "\xe2\x82\xac" "\xf0\xa4\xad\xa2" "ef";
+  constexpr const char* INVALID = "A" "\xc2\x20" "ef";
+  constexpr const char* INCOMPLETE = "A" "\xc2";
   wchar_t out[4];
 
-  const char* valid = "A" "\xc2\xa2" "\xe2\x82\xac" "\xf0\xa4\xad\xa2" "ef";
+  const char* valid = VALID;
   ASSERT_EQ(4U, mbsrtowcs(out, &valid, 4, ps));
   ASSERT_EQ(L'A', out[0]);
   ASSERT_EQ(static_cast<wchar_t>(0x00a2), out[1]);
@@ -362,15 +365,27 @@
   // Check that valid has advanced to the end of the string.
   ASSERT_EQ(nullptr, valid);
 
-  const char* invalid = "A" "\xc2\x20" "ef";
+  const char* invalid = INVALID;
   ASSERT_EQ(static_cast<size_t>(-1), mbsrtowcs(out, &invalid, 4, ps));
   EXPECT_EQ(EILSEQ, errno);
   ASSERT_EQ('\xc2', *invalid);
 
-  const char* incomplete = "A" "\xc2";
+  const char* incomplete = INCOMPLETE;
   ASSERT_EQ(static_cast<size_t>(-1), mbsrtowcs(out, &incomplete, 2, ps));
   EXPECT_EQ(EILSEQ, errno);
   ASSERT_EQ('\xc2', *incomplete);
+
+  // If dst is null, *src shouldn't be updated.
+  // https://code.google.com/p/android/issues/detail?id=166381
+  const char* mbs = VALID;
+  EXPECT_EQ(6U, mbsrtowcs(nullptr, &mbs, 0, ps));
+  EXPECT_EQ(VALID, mbs);
+  mbs = INVALID;
+  EXPECT_EQ(static_cast<size_t>(-1), mbsrtowcs(nullptr, &mbs, 0, ps));
+  EXPECT_EQ(INVALID, mbs);
+  mbs = INCOMPLETE;
+  EXPECT_EQ(static_cast<size_t>(-1), mbsrtowcs(nullptr, &mbs, 0, ps));
+  EXPECT_EQ(INCOMPLETE, mbs);
 }
 
 TEST(wchar, mbsrtowcs) {