diff --git a/scripts/mbedtls_dev/build_tree.py b/scripts/mbedtls_dev/build_tree.py
index f506c41..fa309d3 100644
--- a/scripts/mbedtls_dev/build_tree.py
+++ b/scripts/mbedtls_dev/build_tree.py
@@ -26,9 +26,9 @@
     if root is None:
         root = guess_project_root()
     if looks_like_tf_psa_crypto_root(root):
-        return "core"
+        return os.path.join(root, "core")
     elif looks_like_mbedtls_root(root):
-        return "library"
+        return os.path.join(root, "library")
     else:
         raise Exception('Neither Mbed TLS nor TF-PSA-Crypto source tree found')