diff --git a/.github/workflows/pr-builder.yml b/.github/workflows/pr-builder.yml
index f6b689b8..597f2415 100644
--- a/.github/workflows/pr-builder.yml
+++ b/.github/workflows/pr-builder.yml
@@ -38,6 +38,15 @@ jobs:
       uses: actions/setup-node@v1
       with:
         node-version: ${{ matrix.node-version }}
+    
+    - name: Install dependencies
+      run: |
+        yarn
+        if ! git diff --exit-code > /dev/null 2>&1; then
+          echo "ERR! There are modified files after running yarn."
+          git status -s
+          exit 1
+        fi
 
     - name: Build
       run: |