diff --git a/du3/program.c b/du3/program.c
index 40802fa..b362777 100644
--- a/du3/program.c
+++ b/du3/program.c
@@ -31,6 +31,9 @@ int check(char* input) {
     char znak;
     int a=0;
      a=sscanf(input, " %lf %c %lf = %lf ", &number1, &znak, &number2, &givenResult);
+     if (input == NULL || strlen(input) == 0 || strspn(input, " \t\n") == strlen(input)) {
+        return 0;
+             }
        if (a != 4) {
         return 0;
         }else {