diff --git a/BUILDING.md b/BUILDING.md
index 4517a66ab29..05d378478ec 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -110,6 +110,7 @@ thus to run without postgresql. To build it simply run:
 
     mvn clean package -am -pl packages/system-test
 
+You will need the `patch` utility for it to succeed.
 This entails a completely self-contained dCache instance in
 `packages/system-test/target/dcache`. It can be started using: