diff --git a/src/chart.tsx b/src/chart.tsx
index 1548cf2..0cccbec 100644
--- a/src/chart.tsx
+++ b/src/chart.tsx
@@ -16,8 +16,8 @@ export interface TypeChartChartProps<TType extends GeoChartType> {
   data?: GeoChartData<TType>;
   options?: GeoChartOptions;
   redraw?: boolean;
-  handleSliderXChanged?: (value: number | number[]) => {};
-  handleSliderYChanged?: (value: number | number[]) => {};
+  handleSliderXChanged?: (value: number | number[]) => void;
+  handleSliderYChanged?: (value: number | number[]) => void;
 }
 
 /**