diff --git a/setup.py b/setup.py
index deaf58f..cec9839 100755
--- a/setup.py
+++ b/setup.py
@@ -44,7 +44,8 @@ def parse_version(fpath):
 
 def static_parse(varname, fpath):
     """
-    Statically parse the a constant variable from a python file
+    Statically parse the a constant variable from a python file.
+    Raise an error if the variable is not a constant.
     """
     import ast
 
@@ -55,10 +56,13 @@ def static_parse(varname, fpath):
     pt = ast.parse(sourcecode)
 
     class StaticVisitor(ast.NodeVisitor):
-        def visit_Assign(self, node):
+        def visit_Assign(self, node: ast.Assign):
             for target in node.targets:
                 if getattr(target, "id", None) == varname:
-                    self.static_value = node.value.s
+                    value: ast.expr = node.value
+                    if not isinstance(value, ast.Constant):
+                        raise ValueError("variable {!r} is not a constant".format(varname))
+                    self.static_value = value.value
 
     visitor = StaticVisitor()
     visitor.visit(pt)