diff --git a/src/scale/helper.ts b/src/scale/helper.ts
index 3302b165ee..ef35160dd5 100644
--- a/src/scale/helper.ts
+++ b/src/scale/helper.ts
@@ -128,7 +128,7 @@ export function contain(val: number, extent: [number, number]): boolean {
 
 export function normalize(val: number, extent: [number, number]): number {
     if (extent[1] === extent[0]) {
-        return 0.5;
+        return isNaN(val) ? NaN : 0.5;
     }
     return (val - extent[0]) / (extent[1] - extent[0]);
 }