diff --git a/mbuild/packing.py b/mbuild/packing.py
index 375a2793d..5660ec2f4 100755
--- a/mbuild/packing.py
+++ b/mbuild/packing.py
@@ -172,10 +172,10 @@ def fill_box(compound, n_compounds=None, box=None, density=None, overlap=0.2,
     try:
         iter(fix_orientation[0])
     except:
-        warn('fix_orientation can be given as an iterable of True/False values
-             for each compound to fix rotations about each x,y,z axis individually.
-             Using a single instance of True/False defaults to (True,True,True)
-             and (Fale,False,False) respectively')
+        warnings.warn("fix_orientation can be given as an iterable of True/False values "
+             "for each compound to fix rotations about each x,y,z axis individually. "
+             "Using a single instance of True/False defaults to (True,True,True) "
+             "and (Fale,False,False) respectively")
 
     if compound is not None and n_compounds is not None:
         if len(compound) != len(n_compounds):