diff --git a/blup/frame.py b/blup/frame.py
index 2d339b8..7833841 100644
--- a/blup/frame.py
+++ b/blup/frame.py
@@ -122,6 +122,10 @@ class Frame(object):
 			for v in value:
 				if v < 0 or v >= self.__dimension.depth:
 					raise ValueError('pixel value not in depth range')
+		if x < 0 or x >= self.dimension.width:
+		    raise ValueError('invalid x coordinate')
+		if y < 0 or y >= self.dimension.height:
+		    raise ValueError('invalid y coordinate')
 		self.__pixels[y][x] = value