diff --git a/README_SETUP.md b/README_SETUP.md
index 19a62bda4144550d18f2cd40ea43bbc8431ca903..554cd5dacbdffb637227e6be2b18ed1dad7877dc 100644
--- a/README_SETUP.md
+++ b/README_SETUP.md
@@ -12,6 +12,7 @@
 * libpam (if PAM authentication is required)
 * unzip
 * openpyxl (for XLS/ODS export)
+* openssl (if a custom TLS certificate is required)
 
 ### Install the requirements on Debian
 On Debian, the required packages can be installed with: