diff --git a/appveyor.yml b/appveyor.yml
index b53de00..489d43c 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -19,16 +19,16 @@ environment:
       CC: clang
       CXX: clang++
       CMAKE_ARGS: -DUSE_MBEDTLS=ON
-      PREBUILD_CMD: sudo apt install libmbedtls-dev
+      PREBUILD_CMD: sudo apt install -y libmbedtls-dev
     - APPVEYOR_BUILD_WORKER_IMAGE: Ubuntu
       CC: gcc
       CXX: g++
       CMAKE_ARGS: -DUSE_MBEDTLS=ON
-      PREBUILD_CMD: sudo apt install libmbedtls-dev
+      PREBUILD_CMD: sudo apt install -y libmbedtls-dev
 
 before_build:
   - 'git submodule update --init --recursive'
-  - 'eval "$PREBUILD_CMD"'
+  - sh: 'eval "$PREBUILD_CMD"'
 
 build_script:
   - 'mkdir build_debug'