--- a/src/b5i2iso.c
+++ b/src/b5i2iso.c
@@ -1,3 +1,4 @@
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -45,6 +46,10 @@
    }
 
 fsource = fopen(argv[1],"rb");
+if (fsource == NULL) {
+  printf("can't open %s: %s\n", argv[1], strerror(errno));
+  exit(EXIT_FAILURE);
+}
 fdest = fopen(destfilename,"wb");
 
 fseek(fsource, 2352, SEEK_CUR);