diff --git a/index.js b/index.js
index 0d9b731..3e73f58 100644
--- a/index.js
+++ b/index.js
@@ -28,7 +28,7 @@ export default class FlatQueue {
         this.values = fixed ? /** @type {number[]} */ (/** @type {unknown} */ (new ValuesArray(capacity))) : [];
 
         /** Maximum number of items the queue can hold; `Infinity` for regular-array queues, which grow on demand. */
-        this.capacity = capacity;
+        this.capacity = fixed ? this.ids.length : Infinity;
 
         /** Number of items in the queue. */
         this.length = 0;
diff --git a/test.js b/test.js
index 8fa2ead..4107054 100644
--- a/test.js
+++ b/test.js
@@ -4,6 +4,27 @@ import assert from 'node:assert/strict';
 
 import FlatQueue from './index.js';
 
+test('does not push beyond the storage allocated for a fractional capacity', () => {
+    for (const capacity of [1, 1.5, 1.9]) {
+        const queue = new FlatQueue(capacity);
+        queue.push(42, 1);
+        assert.throws(() => queue.push(43, 0), RangeError);
+        assert.equal(queue.capacity, 1);
+        assert.equal(queue.length, 1);
+        assert.equal(queue.pop(), 42);
+        assert.equal(queue.pop(), undefined);
+    }
+});
+
+test('does not push when the allocated capacity is zero', () => {
+    for (const capacity of [0, 0.5]) {
+        const queue = new FlatQueue(capacity);
+        assert.throws(() => queue.push(42, 1), RangeError);
+        assert.equal(queue.capacity, 0);
+        assert.equal(queue.length, 0);
+    }
+});
+
 const data = [];
 for (let i = 0; i < 100; i++) {
     data.push(Math.floor(100 * Math.random()));