From c74cfa3c5d3486bcfaf76a544b7b6888c757f74b Mon Sep 17 00:00:00 2001
From: Grant Sanderson <grant@3blue1brown.com>
Date: Sun, 15 Dec 2019 08:44:57 -0800
Subject: [PATCH] Add error message for ApplyFunction

---
 manimlib/animation/transform.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/manimlib/animation/transform.py b/manimlib/animation/transform.py
index 32871c03..ef07090c 100644
--- a/manimlib/animation/transform.py
+++ b/manimlib/animation/transform.py
@@ -230,7 +230,10 @@ class ApplyFunction(Transform):
         super().__init__(mobject, **kwargs)
 
     def create_target(self):
-        return self.function(self.mobject.copy())
+        target = self.function(self.mobject.copy())
+        if not isinstance(target, Mobject):
+            raise Exception("Functions passed to ApplyFunction must return object of type Mobject")
+        return target
 
 
 class ApplyMatrix(ApplyPointwiseFunction):