Why does my str3 print weird characters in the last index

Question

#include <stdio.h>
#include <string.h>
int main(void)
{
    char str1[50]="Hello",str2[50]="World";
    char str3[strlen(str1)+strlen(str2)];
    int var=0;
    for (int i=0;i<strlen(str1);i++)
    {
        printf("%d\n",i);
        str3[i]=str1[i];
    }
    for (int i=strlen(str1);i<strlen(str1)+strlen(str2);i++)
    {
        printf("%d,%d\n",i,var);
        str3[i]=str2[var];
        var++;    
    }
    printf("%s",str3);
}

str3 returns HelloWorld plus an unknown character i tried changing the loop variable and the length of the string