diff --git a/.github/workflows/action.yml b/.github/workflows/action.yml
index b30a3048..1e9e4686 100644
--- a/.github/workflows/action.yml
+++ b/.github/workflows/action.yml
@@ -17,7 +17,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v1
         with:
-          node-version: '12'
+          node-version: 12
       - run: npm install -g npm
       - run: npm install
       - run: npm run build
@@ -31,12 +31,24 @@ jobs:
       - name: Check if build left artifacts
         run: git diff --exit-code
 
-  unit_test:
-    name: Unit tests
+  unit_tests_ubuntu:
+    name: Unit tests (Ubuntu)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v1
-        with: {node-version: '12'}
+        with:
+          node-version: 12
       - run: npm ci
       - run: npm test
+
+  unit_tests_windows:
+    name: Unit tests (Windows)
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 12
+      - run: npm install --production
+      - run: npm test
diff --git a/README.md b/README.md
index f232fb36..d24b36a8 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,35 @@ and Erlang/OTP.
 
 **Note** *: prior to 23, Windows builds are only available for minor versions, e.g. 21.0, 21.3, 22.0, etc.
 
+### Self-hosted runners
+
+Self-hosted runners need to set env. variable `ImageOS` to one of the following, since the action
+uses that to download assets:
+
+| ImageOS  | Operating system
+|-         |-
+| ubuntu16 | ubuntu-16.04
+| ubuntu18 | ubuntu-18.04
+| ubuntu20 | ubuntu-20.04
+| win16    | windows-2016
+| win19    | windows-2019
+
+as per the following example:
+
+```yaml
+...
+
+jobs:
+  test:
+    runs-on: self-hosted
+    env:
+      ImageOS: ubuntu20 # equivalent to runs-on ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: erlef/setup-beam@v1
+        ...
+```
+
 ### Basic example (Erlang/OTP + Elixir, on Ubuntu)
 
 ```yaml
diff --git a/__tests__/setup-beam.test.js b/__tests__/setup-beam.test.js
index 394b03a2..7dfca904 100644
--- a/__tests__/setup-beam.test.js
+++ b/__tests__/setup-beam.test.js
@@ -136,7 +136,7 @@ async function testOTPVersions() {
 
     spec = '23.2.x'
     osVersion = 'windows-2016'
-    expected = '23.2.7.4'
+    expected = '23.2.7'
     got = await setupBeam.getOTPVersion(spec, osVersion)
     assert.deepStrictEqual(got, expected)
 
diff --git a/dist/index.js b/dist/index.js
index 6a97199b..3b76cec8 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -5001,8 +5001,19 @@ function getRunnerOSVersion() {
     win16: 'windows-2016',
     win19: 'windows-2019',
   }
+  const containerFromEnvImageOS = ImageOSToContainer[process.env.ImageOS]
 
-  return ImageOSToContainer[process.env.ImageOS]
+  if (!containerFromEnvImageOS) {
+    throw new Error(
+      "Tried to map a target OS from env. variable 'ImageOS', but failed. If you're using a " +
+        "self-hosted runner, you should set 'env': 'ImageOS': ... to one of the following: " +
+        "['" +
+        `${Object.keys(ImageOSToContainer).join("', '")}` +
+        "']",
+    )
+  }
+
+  return containerFromEnvImageOS
 }
 
 async function get(url0, pageIdxs) {
diff --git a/src/setup-beam.js b/src/setup-beam.js
index 125f874a..0dfee5fa 100644
--- a/src/setup-beam.js
+++ b/src/setup-beam.js
@@ -345,8 +345,19 @@ function getRunnerOSVersion() {
     win16: 'windows-2016',
     win19: 'windows-2019',
   }
+  const containerFromEnvImageOS = ImageOSToContainer[process.env.ImageOS]
 
-  return ImageOSToContainer[process.env.ImageOS]
+  if (!containerFromEnvImageOS) {
+    throw new Error(
+      "Tried to map a target OS from env. variable 'ImageOS', but failed. If you're using a " +
+        "self-hosted runner, you should set 'env': 'ImageOS': ... to one of the following: " +
+        "['" +
+        `${Object.keys(ImageOSToContainer).join("', '")}` +
+        "']",
+    )
+  }
+
+  return containerFromEnvImageOS
 }
 
 async function get(url0, pageIdxs) {