diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e6bad3e..3ed0e76 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        env: [ '', 'RUST_PROTOBUF=1' ]
+        env: [ 'RUST_PROTOBUF=0', 'RUST_PROTOBUF=1' ]
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        env: [ '', 'RUST_PROTOBUF=1' ]
+        env: [ 'RUST_PROTOBUF=0', 'RUST_PROTOBUF=1' ]
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
@@ -67,7 +67,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        env: [ '', 'RUST_PROTOBUF=1' ]
+        env: [ 'RUST_PROTOBUF=0', 'RUST_PROTOBUF=1' ]
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1