Crashed when compiled as dynamic library for C

[Zachary]

Hi all, I am a newbie to Nim and I have tried to write Nim as dynamic library 
for C, but I got some problems.

person.nim: 
    
    
    import strutils
    
    type
        Person = object of RootObj
            name*: string
            age*: Natural
    
    proc createPerson(n: int):ref Person {.cdecl, exportc: "createPerson", 
dynlib.} =
        var p = new(Person)
        p.age = 25
        p.name = newString(20)
        # p.name = format("jiazequn#$1", n)
        return p
    
    proc printPerson(p:ref Person) {.cdecl, exportc: "printPerson", dynlib.} =
        echo format("name is $1 and age is $2", p.name, p.age)
    
    proc nim_main() {.cdecl, exportc: "nim_main", dynlib.} =
        var num = 0
        while true:
            num += 1
            var p = createPerson(num)
            if num mod 10000 == 0:
                printPerson(p)
                echo num
    
    
    
    Run

main.c
    
    
    #include <stdio.h>
    extern void nim_main();
    
    int main()
    {
        nim_main();
        return 0;
    }
    
    
    Run

But I got the following errors when running after a while: 
    
    
    name is  and age is 25
    2180000
    Traceback (most recent call last)
    /Users/zachary/Developer/nim-demo/person.nim(24) nim_main
    /Users/zachary/Developer/nim-demo/person.nim(16) printPerson
    /usr/local/Cellar/nim/1.0.0/nim/lib/pure/strutils.nim(2751) format
    /usr/local/Cellar/nim/1.0.0/nim/lib/system/gc.nim(439) newObj
    /usr/local/Cellar/nim/1.0.0/nim/lib/system/alloc.nim(777) rawAlloc
    SIGSEGV: Illegal storage access. (Attempt to read from nil?)
    
    
    Run

It seems that it is the GC that cause this problem. If I disable the gc using 
GC_disable() at the beginning of nim_main(), no error occurred.

Could you please give me some information about how to get it fixed?

Also my final goal is to move nim_main function into main.c, now it have the 
similar problem.

Thanks!