diff --git a/Dockerfile b/Dockerfile
index 48e26c5..714965d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -6,7 +6,7 @@ ARG MOD_VERSION
 
 RUN \
   DOTNET_JSON=$(curl -sX GET "https://raw.githubusercontent.com/dotnet/core/master/release-notes/releases-index.json") && \
-  if [ -z ${MOD_VERSION+x} ]; then \
+  if [ -z "${MOD_VERSION}" ]; then \
     MOD_VERSION=$(echo "$DOTNET_JSON" | jq -r '."releases-index"[] | select(."support-phase"=="active" or ."support-phase"=="maintenance") | ."latest-sdk"' | tr '\n' '_' | head -c -1); \
   fi && \
   DOTNET_VERSIONS="${MOD_VERSION//_/ }" && \