diff --git a/doc/errors/index.md b/doc/errors/index.md
index 79ac5dac7c..4213a0b4d1 100644
--- a/doc/errors/index.md
+++ b/doc/errors/index.md
@@ -236,3 +236,7 @@ A `WebGLArrayBuffer` must either be of type `ELEMENT_ARRAY_BUFFER` or `ARRAY_BUF
 ### 63
 
 Support for the `OES_element_index_uint` WebGL extension is mandatory for WebGL layers.
+
+### 64
+
+Layer opacity must be a number.
diff --git a/src/ol/layer/Base.js b/src/ol/layer/Base.js
index 2be76892d7..7b646c01dd 100644
--- a/src/ol/layer/Base.js
+++ b/src/ol/layer/Base.js
@@ -6,6 +6,7 @@ import BaseObject from '../Object.js';
 import LayerProperty from './Property.js';
 import {clamp} from '../math.js';
 import {assign} from '../obj.js';
+import {assert} from '../asserts.js';
 
 
 /**
@@ -52,8 +53,11 @@ class BaseLayer extends BaseObject {
      * @type {Object<string, *>}
      */
     const properties = assign({}, options);
+
     properties[LayerProperty.OPACITY] =
-       options.opacity !== undefined ? options.opacity : 1;
+        options.opacity !== undefined ? options.opacity : 1;
+    assert(typeof properties[LayerProperty.OPACITY] === 'number', 64); // Layer opacity must be a number
+
     properties[LayerProperty.VISIBLE] =
        options.visible !== undefined ? options.visible : true;
     properties[LayerProperty.Z_INDEX] = options.zIndex;
@@ -292,6 +296,7 @@ class BaseLayer extends BaseObject {
    * @api
    */
   setOpacity(opacity) {
+    assert(typeof opacity === 'number', 64); // Layer opacity must be a number
     this.set(LayerProperty.OPACITY, opacity);
   }
 
diff --git a/test/spec/ol/layer/layer.test.js b/test/spec/ol/layer/layer.test.js
index f7d6b5f4ee..50f494dc18 100644
--- a/test/spec/ol/layer/layer.test.js
+++ b/test/spec/ol/layer/layer.test.js
@@ -112,6 +112,19 @@ describe('ol.layer.Layer', function() {
       layer.dispose();
     });
 
+    it('throws on non-numeric opacity', function() {
+      function create() {
+        new Layer({
+          source: new Source({
+            projection: 'EPSG:4326'
+          }),
+          opacity: 'foo'
+        });
+      }
+
+      expect(create).to.throwException();
+    });
+
     it('accepts a custom render function', function() {
       let called = false;
       const layer = new Layer({
@@ -511,6 +524,13 @@ describe('ol.layer.Layer', function() {
       expect(layer.getOpacity()).to.be(0.3);
     });
 
+    it('throws on types other than number', function() {
+      function set() {
+        layer.setOpacity('foo');
+      }
+      expect(set).to.throwException();
+    });
+
     it('triggers a change event', function() {
       const listener = sinon.spy();
       layer.on('propertychange', listener);