diff --git a/types/d3-contour/d3-contour-tests.ts b/types/d3-contour/d3-contour-tests.ts
index 588750bf29..2a7444ab19 100644
--- a/types/d3-contour/d3-contour-tests.ts
+++ b/types/d3-contour/d3-contour-tests.ts
@@ -38,7 +38,7 @@ function goldsteinPrice(x: number, y: number) {
 
 let size: [number, number];
 let boolFlag: boolean;
-const thresholdArrayGen: ThresholdArrayGenerator<number> = (values: number[], min: number, max: number) => {
+const thresholdArrayGen: ThresholdArrayGenerator<number> = (values: ArrayLike<number>, min?: number, max?: number) => {
     let thresholds: number[];
     thresholds = [values[1], values[2], values[4]];
     return thresholds;
diff --git a/types/d3-contour/tsconfig.json b/types/d3-contour/tsconfig.json
index d367ee16c7..345a064caa 100644
--- a/types/d3-contour/tsconfig.json
+++ b/types/d3-contour/tsconfig.json
@@ -8,7 +8,7 @@
         "noImplicitAny": true,
         "noImplicitThis": true,
         "strictNullChecks": true,
-        "strictFunctionTypes": false,
+        "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [
             "../"