C :: Warning - Unknown Escape Sequence

I'm doing some file input/output work here in C and received this warning during compilation (GCC). My research indicates that this error is in response to white space or a character cancellation function or something like that. I'm not 100% sure exactly what it means. My code works fine, but the following warning does appear.

Code:
warning: unknown escape sequence: '40'

Here's my code (excluding a bunch of comments at the bottom of the file).

Code:
#include <stdio.h>
int main(void){
FILE *file;
file = fopen("Running Practice.c", "a");
fprintf(file, "Testing...
");
fclose(file);
}

I believe the error I received has to do with either the ' ' I used when appending text to my file, or something to do with there being a space in the file name itself.