diff --git a/optimizer/extlib/nsga2/nsga2_io.c b/optimizer/extlib/nsga2/nsga2_io.c
index 497b3e5377631c98b28fc261f7e126a401ed8787..e776fd206258248274d181ef32bb0ef6fd3c8a8a 100644
--- a/optimizer/extlib/nsga2/nsga2_io.c
+++ b/optimizer/extlib/nsga2/nsga2_io.c
@@ -94,7 +94,7 @@ void write_pop(char* filename, pop* pp, int size)
      int i;
      FILE *fp;
 
-     assert(0 <= size || size <= pp->size);
+     assert(0 <= size && size <= pp->size);
 
      fp = fopen(filename, "w");
      assert(fp != NULL);