diff --git a/libc/bionic/wchar.cpp b/libc/bionic/wchar.cpp
index a507808..021d14b 100644
--- a/libc/bionic/wchar.cpp
+++ b/libc/bionic/wchar.cpp
@@ -250,6 +250,9 @@
     // TODO: UTF-8 support.
     if ((*src)[i] > 0x7f) {
       errno = EILSEQ;
+      if (dst != NULL) {
+        *src = &(*src)[i];
+      }
       return static_cast<size_t>(-1);
     }
     if (dst != NULL) {
diff --git a/tests/wchar_test.cpp b/tests/wchar_test.cpp
index 0a63b00..d5d27ed 100644
--- a/tests/wchar_test.cpp
+++ b/tests/wchar_test.cpp
@@ -58,8 +58,8 @@
 }
 
 TEST(wchar, wcstombs_wcrtombs) {
-  wchar_t chars[] = { L'h', L'e', L'l', L'l', L'o', 0 };
-  wchar_t bad_chars[] = { L'h', L'i', 666, 0 };
+  const wchar_t chars[] = { L'h', L'e', L'l', L'l', L'o', 0 };
+  const wchar_t bad_chars[] = { L'h', L'i', 666, 0 };
   const wchar_t* src;
   char bytes[BUFSIZ];