diff --git a/mslice/models/labels.py b/mslice/models/labels.py
index 3e613cf26..f4c75ddb1 100644
--- a/mslice/models/labels.py
+++ b/mslice/models/labels.py
@@ -24,7 +24,7 @@ def is_momentum(unit):
 
 
 def is_energy(unit):
-    return unit == 'DeltaE'
+    return unit == DELTA_E_LABEL 
 
 
 def are_units_equivalent(unit_lhs, unit_rhs):