diff --git a/ceviche/sources.py b/ceviche/sources.py
index 7802e59..ce8a9ef 100644
--- a/ceviche/sources.py
+++ b/ceviche/sources.py
@@ -1,4 +1,5 @@
 import autograd.numpy as npa
+from constants import C_0
 
 """ Source functions go here.  For now it's just TFSF """