diff --git a/src/quantile.js b/src/quantile.js
index 75d9726..f4a8a48 100644
--- a/src/quantile.js
+++ b/src/quantile.js
@@ -38,8 +38,8 @@ export function quantileIndex(values, p, valueof = number) {
   if (p <= 0) return minIndex(numbers);
   if (p >= 1) return maxIndex(numbers);
   var numbers,
-      index = Uint32Array.from(values, (_, i) => i),
-      j = numbers.length - 1,
+      index = Uint32Array.from(values, (_, i) => i).filter(i => !isNaN(numbers[i])),
+      j = index.length - 1,
       i = Math.floor(j * p);
   quickselect(index, i, 0, j, (i, j) => ascendingDefined(numbers[i], numbers[j]));
   i = greatest(index.subarray(0, i + 1), (i) => numbers[i]);
diff --git a/test/median-test.js b/test/median-test.js
index 78d93f3..3818121 100644
--- a/test/median-test.js
+++ b/test/median-test.js
@@ -98,6 +98,11 @@ it("median(array, f) uses the undefined context", () => {
   assert.deepStrictEqual(results, [undefined, undefined]);
 });
 
+it("medianIndex(array) excludes missing values from the median rank", () => {
+  assert.strictEqual(medianIndex([0, null, 100]), 0);
+  assert.strictEqual(medianIndex([undefined, 0, 100]), 1);
+});
+
 it("medianIndex(array) returns the index", () => {
   assert.deepStrictEqual(medianIndex([1, 2]), 0);
   assert.deepStrictEqual(medianIndex([1, 2, 3]), 1);
diff --git a/test/quantile-test.js b/test/quantile-test.js
index 4b0d0ec..288b669 100644
--- a/test/quantile-test.js
+++ b/test/quantile-test.js
@@ -122,6 +122,16 @@ it("quantileIndex(array, 0.5) handles undefined values", () => {
   assert.deepStrictEqual(quantileIndex([1, 1, 1, null, 2, 3, 3, 3], 0.5, (d) => d), 4);
 });
 
+it("quantileIndex(array, p) excludes missing values from the quantile rank", () => {
+  for (const missing of [null, undefined, NaN]) {
+    assert.strictEqual(quantileIndex([0, missing, 100], 0.5), 0);
+    assert.strictEqual(quantileIndex([missing, 0, 100], 0.5), 1);
+    assert.strictEqual(quantileIndex([0, 100, missing], 0.5), 0);
+  }
+  assert.strictEqual(quantileIndex([{v: 0}, {}, {v: 100}], 0.5, d => d.v), 0);
+  assert.strictEqual(quantileIndex([null, undefined, NaN], 0.5), -1);
+});
+
 it("quantileIndex(array, 0.5) returns the first of equivalent values", () => {
   assert.deepStrictEqual(quantileIndex([1, 1, 1, 2, 2, 3, 3, 3], 0.5), 4);
 });