diff --git a/tools/psa/README.md b/tools/psa/README.md
index 3719a700d7..5de016913b 100644
--- a/tools/psa/README.md
+++ b/tools/psa/README.md
@@ -51,6 +51,9 @@ optional arguments:
   -x ..., --extra ...  additional build parameters
 ```
 
+* The script must be run from the mbed-os folder via `tools/psa/release.py`,
+  otherwise the list of available tests will not be accurate and the test
+  partitions will not be properly generated.
 * When `MCU ` is not specified, the script compiles all the images for all the targets.
 * When `-t/--tc` is not specified, the script compiles with the default_toolchain speciified in targets.json.
 * When `-d/--debug` is not specified, the script compiles the images using the release profile.