diff --git a/includes/math-expr.inc b/includes/math-expr.inc
index 325d444..e353370 100644
--- a/includes/math-expr.inc
+++ b/includes/math-expr.inc
@@ -101,6 +101,7 @@ class ctools_math_expr {
         // make the variables a little more accurate
         $this->v['pi'] = pi();
         $this->v['e'] = exp(1);
+        $this->fb = array_merge($this->fb, variable_get('ctools_math_expression_functions', array()));
     }
 
     function e($expr) {