how to make a macro work as a single line if stmt without braces

I have this macro:

#define WARN(...)                                       \
    do {                                                \
        fprintf(stderr, "%s#%d: ", __FILE__, __LINE__); \
        fprintf(stderr, __VA_ARGS__);                   \
    } while (0);

which I use like this:

total++;
if (failed) {
    WARN("failed because...");
} else
    ok++;

I would prefer to be able to write this instead:

total++;
if (failed)
    WARN("failed because...");
else
    ok++;

but doing so results in a compiler error:

error: 'else' without a previous 'if'