diff --git a/library/include/mobilisis/configuration.h b/library/include/mobilisis/configuration.h
index 3c4966e..51970b8 100644
--- a/library/include/mobilisis/configuration.h
+++ b/library/include/mobilisis/configuration.h
@@ -30,6 +30,7 @@
 using namespace std;
 using namespace rapidjson;
 
+class Calculator;
 class Configuration
 {
 public: