diff --git a/.travis.yml b/.travis.yml
index e500b38..4c6a932 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,8 +12,7 @@ os:
   - osx
 
 env:
-  GO111MODULE=on
-  GOPROXY=https://proxy.golang.org
+  GO111MODULE=on GOPROXY=https://proxy.golang.org GOFLAGS=-mod=readonly
 
 cache:
   directories:
diff --git a/appveyor.yml b/appveyor.yml
index 598ff4b..ca41f62 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -14,6 +14,7 @@ environment:
   GOVERSION: 1.11.x
   GO111MODULE: on
   GOPROXY: https://proxy.golang.org
+  GOFLAGS: -mod=readonly
 
 install:
   - set PATH=%GOPATH%\bin;C:\go\bin;%PATH%