diff --git a/library/libc.gmk b/library/libc.gmk
index 746af4d..63707e5 100644
--- a/library/libc.gmk
+++ b/library/libc.gmk
@@ -234,6 +234,7 @@ C_LIB := \
 	stdlib_main_stub.o \
 	stdlib_malloc.o \
 	stdlib_math.o \
+	stdlib_mbtowc.o \
 	stdlib_mkdtemp.o \
 	stdlib_mkstemp.o \
 	stdlib_mktemp.o \
diff --git a/library/stdlib_mbtowc.c b/library/stdlib_mbtowc.c
index 0d592a6..9db5ee6 100644
--- a/library/stdlib_mbtowc.c
+++ b/library/stdlib_mbtowc.c
@@ -37,13 +37,13 @@
 
 /****************************************************************************/
 
-#if defined(defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L))
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
 
 int
 mbtowc(wchar_t *restrict pwc, const char *restrict s, size_t n)
 {
-	/* ZZZ unimplemented */
-	return(-1);
+	errno = EILSEQ;
+	return -1;
 }
 
 #endif