diff --git a/HISTORY b/HISTORY
index 0682c99..d99d6c6 100755
--- a/HISTORY
+++ b/HISTORY
@@ -26,7 +26,7 @@ May. 24 2017
 Fixed a memory leak when applying GC on a forked environment
 
 May. 20 2017
-Improved stability by preventing assigning builtin boolean
+Improved stability by preventing assigning built-in boolean
 
 May. 18 2017
 Fixed some memory potential leak when popped unexpected type of argument
diff --git a/MY-BASIC Quick Reference.pdf b/MY-BASIC Quick Reference.pdf
index 822b310..495e874 100644
Binary files a/MY-BASIC Quick Reference.pdf and b/MY-BASIC Quick Reference.pdf differ
diff --git a/core/my_basic.c b/core/my_basic.c
index af26112..1c72670 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -11705,21 +11705,21 @@ int mb_get_forked_from(struct mb_interpreter_t* s, struct mb_interpreter_t** src
 
 /* Register an API function to a MY-BASIC environment */
 int mb_register_func(struct mb_interpreter_t* s, const char* n, mb_func_t f) {
-	if(!s || !n || !f) return MB_FUNC_ERR;
+	if(!s || !n || !f) return 0;
 
 	return _register_func(s, (char*)n, f, false);
 }
 
 /* Remove an API function from a MY-BASIC environment */
 int mb_remove_func(struct mb_interpreter_t* s, const char* n) {
-	if(!s || !n) return MB_FUNC_ERR;
+	if(!s || !n) return 0;
 
 	return _remove_func(s, (char*)n, false);
 }
 
 /* Remove a reserved API from a MY-BASIC environment */
 int mb_remove_reserved_func(struct mb_interpreter_t* s, const char* n) {
-	if(!s || !n) return MB_FUNC_ERR;
+	if(!s || !n) return 0;
 
 	return _remove_func(s, (char*)n, true);
 }