diff --git a/du3/program.c b/du3/program.c
index 666d100..e0a269e 100644
--- a/du3/program.c
+++ b/du3/program.c
@@ -28,7 +28,7 @@ int main() {
                                 index++;
                                 continue;
                         }
-			if ((isalpha(buffer[index]) || strchr(".+-*/=123456789\n", buffer[index]) == NULL) && !error) {
+			if ((isalpha(buffer[index]) || strchr(".+-*/=0123456789\n", buffer[index]) == NULL) && !error) {
 				error = 1;
 				strcpy(output[output_length], "CHYBA");
 				output_length++;