diff --git a/rtengine/array2D.h b/rtengine/array2D.h
index c79d4b5b5..e79482543 100644
--- a/rtengine/array2D.h
+++ b/rtengine/array2D.h
@@ -157,7 +157,7 @@ public:
 
 	// use with indices
 	T * operator[](int index) {
-        assert(index<y); 
+        assert((index>=0) && (index < y));
 		return ptr[index];
 	}