diff --git a/impl/getusername.cpp b/impl/getusername.cpp
index 35c034fb9d..f9c73280a4 100644
--- a/impl/getusername.cpp
+++ b/impl/getusername.cpp
@@ -43,8 +43,8 @@ BOOL GetUserName(WCHAR_T *lpBuffer, LPDWORD lpnSize)
 	}
 
 	// Get username from system in a thread-safe manner
-	char userName[*lpnSize];
-	int err = getlogin_r(userName, *lpnSize);
+	char userName[L_cuserid];
+	int err = getlogin_r(userName, L_cuserid);
 	// Map errno to Win32 Error Codes
 	if (err != 0) {
 		switch (errno) {
diff --git a/tests/test-getusername.cpp b/tests/test-getusername.cpp
index c7bbc14b47..7e1badee0d 100644
--- a/tests/test-getusername.cpp
+++ b/tests/test-getusername.cpp
@@ -8,7 +8,7 @@
 TEST(GetUserName,simple)
 {
 	// allocate a WCHAR_T buffer to receive username
-	DWORD lpnSize = 64;
+	DWORD lpnSize = L_cuserid;
 	WCHAR_T lpBuffer[lpnSize];
 
 	BOOL result = GetUserName(lpBuffer, &lpnSize);