diff --git a/sparselm/tools.py b/sparselm/tools.py
index d6103f7..98df200 100644
--- a/sparselm/tools.py
+++ b/sparselm/tools.py
@@ -7,7 +7,7 @@
 import numpy as np
 
 
-def constrain_coefficients(indices, high, low=0.0):
+def constrain_coefficients(indices, high=None, low=None):
     """Constrain a fit method to keep coefficients within a specified range.
 
     Decorator to enforce that a fit method fitting a cluster expansion that
@@ -30,8 +30,14 @@ def your_fit_method(X, y):
     """
 
     indices = np.array(indices)
-    high = high * np.ones(len(indices)) if isinstance(high, float) else np.array(high)
-    low = low * np.ones(len(indices)) if isinstance(low, float) else np.array(low)
+    if high is not None:
+        high = high * np.ones(len(indices)) if isinstance(high, float) else np.array(high)
+    else:
+        high = np.inf * np.ones(len(indices))
+    if low is not None:
+        low = low * np.ones(len(indices)) if isinstance(low, float) else np.array(low)
+    else:
+        low = -np.inf * np.ones(len(indices))
 
     def decorate_fit_method(fit_method):
         """Decorate a fit method to constrain "dielectric constant".