diff --git a/src/primitives.py b/src/primitives.py
index d8361b4efb107d979b73f9bc1e462cb5536ff42a..851c769e60dda2800a71d3d263ecd81f01df75d6 100644
--- a/src/primitives.py
+++ b/src/primitives.py
@@ -624,9 +624,18 @@ def quotient(numerator, denominator):
 
 
 # tool functions --------------------------------------------------------------
+global VALID_CONSTANT_CLASSES
+global VALID_OPERANDS
 VALID_CONSTANT_CLASSES = (int, float, complex)
 VALID_OPERANDS = (Expression,)
 
+try:
+    import numpy
+    VALID_CONSTANT_CLASSES += (numpy.number,)
+except ImportError:
+    pass
+
+
 
 
 def is_constant(value):
@@ -639,9 +648,13 @@ def is_valid_operand(value):
 
 
 def register_constant_class(class_):
+    global VALID_CONSTANT_CLASSES
+
     VALID_CONSTANT_CLASSES += (class_,)
 
 def unregister_constant_class(class_):
+    global VALID_CONSTANT_CLASSES
+
     tmp = list(VALID_CONSTANT_CLASSES)
     tmp.remove(class_)
     VALID_CONSTANT_CLASSES = tuple(tmp)
@@ -660,6 +673,3 @@ def is_zero(value):
 
 
 
-
-
-