File opens but write fails

The following is a functions that writes a line to a diagnostic file...

Code:

void Write_to_Diagnostic_Log(const char * const Log_String, const char * const sDiagnostic_Log_Path) {
  FILE *fpDiagnostic_Log;              // Pointer to the Diagnostic Log File

  fpDiagnostic_Log = fopen(sDiagnostic_Log_Path, "a");

  if (fpDiagnostic_Log == NULL) printf("\n\n%s\n\n", sDiagnostic_Log_Path);

  fprintf(fpDiagnostic_Log, "%s", Log_String);

  fclose(fpDiagnostic_Log);  
}

The file opens correctly, but nothing prints into it. the fprintf statement returns -1, but I've double checked the fopen and it succeeds...

What gives?

bah... nevermind... I forgot to use a malloc somewhere else and I guess my memory got messed up...