diff --git a/install.sh b/install.sh
index 46ca3568b7..6b6c664e8d 100755
--- a/install.sh
+++ b/install.sh
@@ -153,6 +153,9 @@ setup_env() {
                 if [ -z "${K3S_TOKEN}" ] && [ -z "${K3S_CLUSTER_SECRET}" ]; then
                     fatal "Defaulted k3s exec command to 'agent' because K3S_URL is defined, but K3S_TOKEN or K3S_CLUSTER_SECRET is not defined."
                 fi
+                if ! echo "${K3S_URL}" | grep -q "^https://"; then
+                    fatal "Only https:// URLs are supported for K3S_URL (have ${K3S_URL})"
+                fi
                 CMD_K3S=agent
             fi
         ;;