diff --git a/src/if_ruby.c b/src/if_ruby.c
index 2295fa5..6135c5d 100644
--- a/src/if_ruby.c
+++ b/src/if_ruby.c
@@ -869,7 +869,7 @@
     long	n = NUM2LONG(num);
     aco_save_T	aco;
 
-    if (line != NULL) {
+    if (line == NULL) {
 	rb_raise(rb_eIndexError, "NULL line");
     }
     else if (n >= 0 && n <= buf->b_ml.ml_line_count)