[plt-scheme] malloc/free inside MzScheme

Inside PLT MzScheme, Chapter 3 Memory Allocation:
MzScheme uses both malloc and allocation functions provided by a garbage collector. Embedding/extension C/C++ code may use either allocation method, ...

Here is my program that embeds MzScheme v300.3. The intention is to get error message as UTF-16 string. 

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#include <string.h>
#include <stdlib.h>
#include "PLTHOME/include/scheme.h"

Scheme_Env *e;

long write_errmsg_fun(Scheme_Output_Port *port, const char *buffer, long offset, long size, int rarely_block)
{
	int i, n;
	unsigned short *s;
	if (!(size == 0L || size==1L && buffer[offset]=='\12'))
	{
		s = (unsigned short *)malloc( scheme_utf8_decode( buffer, offset, size, NULL, 0, -1, NULL, 1, 0));
		n = scheme_utf8_decode( buffer, offset, size, s, 0, -1, NULL, 1, 0);
		for (i=0;i<n;i++)
			printf("%c", s[i]);
		free( s);//error here
	}
	return size;
}

int char_ready_fun(Scheme_Object *port)
{return 1;}

void close_fun(Scheme_Object *port)
{}

void need_wakeup_fun(Scheme_Object *port, void *fds)
{}

Scheme_Object * make_error_port( void)
{
	Scheme_Object *tmp, *result;
	MZ_GC_DECL_REG(1);
	tmp = scheme_make_utf8_string("<error>");
	MZ_GC_VAR_IN_REG(0, tmp);
	MZ_GC_REG();
	result = (Scheme_Object *) scheme_make_output_port(NULL, NULL,
		tmp, NULL, 
		&write_errmsg_fun, 
		&char_ready_fun, 
		&close_fun, 
		&need_wakeup_fun, 
		NULL, NULL, 0);
	MZ_GC_UNREG();
	return result;
}

void init( void)
{
	scheme_set_stack_base( NULL, 1);
	scheme_make_stderr = &make_error_port;
	e = scheme_basic_env();
}

int main(int argc, char* argv[])
{
	init( );
	if (scheme_setjmp(scheme_error_buf)) {
		return -1;
	} else {
		scheme_apply(scheme_eval_string("read-eval-print-loop", e), 0, NULL);
	}
	return 0;
}

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I tried this program both under Windows/MSVC and linux/gcc. Whenever the 

		free( s);//error here

was executed, a fatal error happens and the program exits abnormally.
Could anyone help telling what's wrong? Many thanks.

Sincerely,
Chongkai Zhu