diff --git a/apps/calculation/expression_field.h b/apps/calculation/expression_field.h
index 2844046286..33477a8744 100644
--- a/apps/calculation/expression_field.h
+++ b/apps/calculation/expression_field.h
@@ -13,6 +13,8 @@ class ExpressionField : public ::ExpressionField {
   }
 protected:
   bool handleEvent(Ion::Events::Event event) override;
+private:
+  bool fieldHasOnlyAMinus() const;
 };
 
 }