diff --git a/libc/Android.mk b/libc/Android.mk
index 90b0d4e..864287a 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -336,6 +336,7 @@
     upstream-openbsd/lib/libc/gen/time.c \
     upstream-openbsd/lib/libc/gen/tolower_.c \
     upstream-openbsd/lib/libc/gen/toupper_.c \
+    upstream-openbsd/lib/libc/locale/mbtowc.c \
     upstream-openbsd/lib/libc/locale/wcscoll.c \
     upstream-openbsd/lib/libc/locale/wcsxfrm.c \
     upstream-openbsd/lib/libc/stdio/asprintf.c \
diff --git a/libc/bionic/wchar.cpp b/libc/bionic/wchar.cpp
index f921aa0..674dc3a 100644
--- a/libc/bionic/wchar.cpp
+++ b/libc/bionic/wchar.cpp
@@ -164,16 +164,12 @@
 
 size_t mbrtowc(wchar_t* pwc, const char* s, size_t n, mbstate_t* /*ps*/) {
   if (s == NULL) {
-    s   = "";
-    pwc = NULL;
+    return 0;
   }
   if (n == 0) {
-    if (pwc) {
-      *pwc = 0;
-      return 0;
-    }
+    return 0;
   }
-  if (pwc) {
+  if (pwc != NULL) {
     *pwc = *s;
   }
   return (*s != 0);
diff --git a/libc/upstream-openbsd/lib/libc/locale/mbtowc.c b/libc/upstream-openbsd/lib/libc/locale/mbtowc.c
new file mode 100644
index 0000000..920f4bf
--- /dev/null
+++ b/libc/upstream-openbsd/lib/libc/locale/mbtowc.c
@@ -0,0 +1,50 @@
+/*	$OpenBSD: mbtowc.c,v 1.2 2012/12/05 23:20:00 deraadt Exp $ */
+
+/*-
+ * Copyright (c) 2002-2004 Tim J. Robbins.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+
+#include <stdlib.h>
+#include <string.h>
+#include <wchar.h>
+#include <errno.h>
+
+int
+mbtowc(wchar_t * __restrict pwc, const char * __restrict s, size_t n)
+{
+	static mbstate_t mbs;
+	size_t rval;
+
+	if (s == NULL) {
+		/* No support for state dependent encodings. */
+		memset(&mbs, 0, sizeof(mbs));
+		return (0);
+	}
+	rval = mbrtowc(pwc, s, n, &mbs);
+	if (rval == (size_t)-1 || rval == (size_t)-2)
+		return (-1);
+	return ((int)rval);
+}
diff --git a/tests/wchar_test.cpp b/tests/wchar_test.cpp
index eff845a..3c38f1f 100644
--- a/tests/wchar_test.cpp
+++ b/tests/wchar_test.cpp
@@ -174,3 +174,41 @@
   ASSERT_EQ(&haystack[10], wcswcs(haystack, good_needle));
   ASSERT_EQ(NULL, wcswcs(haystack, bad_needle));
 }
+
+TEST(wchar, mbtowc) {
+  wchar_t out[8];
+
+  out[0] = 'x';
+  ASSERT_EQ(0, mbtowc(out, "hello", 0));
+  ASSERT_EQ('x', out[0]);
+
+  ASSERT_EQ(0, mbtowc(out, "hello", 0));
+  ASSERT_EQ(0, mbtowc(out, "", 0));
+  ASSERT_EQ(1, mbtowc(out, "hello", 1));
+  ASSERT_EQ(L'h', out[0]);
+
+  ASSERT_EQ(0, mbtowc(NULL, "hello", 0));
+  ASSERT_EQ(0, mbtowc(NULL, "", 0));
+  ASSERT_EQ(1, mbtowc(NULL, "hello", 1));
+
+  ASSERT_EQ(0, mbtowc(NULL, NULL, 0));
+}
+
+TEST(wchar, mbrtowc) {
+  wchar_t out[8];
+
+  out[0] = 'x';
+  ASSERT_EQ(0U, mbrtowc(out, "hello", 0, NULL));
+  ASSERT_EQ('x', out[0]);
+
+  ASSERT_EQ(0U, mbrtowc(out, "hello", 0, NULL));
+  ASSERT_EQ(0U, mbrtowc(out, "", 0, NULL));
+  ASSERT_EQ(1U, mbrtowc(out, "hello", 1, NULL));
+  ASSERT_EQ(L'h', out[0]);
+
+  ASSERT_EQ(0U, mbrtowc(NULL, "hello", 0, NULL));
+  ASSERT_EQ(0U, mbrtowc(NULL, "", 0, NULL));
+  ASSERT_EQ(1U, mbrtowc(NULL, "hello", 1, NULL));
+
+  ASSERT_EQ(0U, mbrtowc(NULL, NULL, 0, NULL));
+}