diff --git a/Libraries/LibC/stdlib.cpp b/Libraries/LibC/stdlib.cpp
index 04a02b0cb0..90e4c639d0 100644
--- a/Libraries/LibC/stdlib.cpp
+++ b/Libraries/LibC/stdlib.cpp
@@ -796,7 +796,7 @@ size_t mbstowcs(wchar_t*, const char*, size_t)
     ASSERT_NOT_REACHED();
 }
 
-size_t mbtowc(wchar_t* wch, const char* data, size_t data_size)
+int mbtowc(wchar_t* wch, const char* data, size_t data_size)
 {
     // FIXME: This needs a real implementation.
     UNUSED_PARAM(data_size);
diff --git a/Libraries/LibC/stdlib.h b/Libraries/LibC/stdlib.h
index fe4fe37a65..089d47b389 100644
--- a/Libraries/LibC/stdlib.h
+++ b/Libraries/LibC/stdlib.h
@@ -74,7 +74,7 @@ int mkstemp(char*);
 char* mkdtemp(char*);
 void* bsearch(const void* key, const void* base, size_t nmemb, size_t size, int (*compar)(const void*, const void*));
 size_t mbstowcs(wchar_t*, const char*, size_t);
-size_t mbtowc(wchar_t*, const char*, size_t);
+int mbtowc(wchar_t*, const char*, size_t);
 int wctomb(char*, wchar_t);
 size_t wcstombs(char*, const wchar_t*, size_t);
 char* realpath(const char* pathname, char* buffer);