diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 0429bdd..1f9389b 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -13,8 +13,10 @@
 ## Tools
 OPENSSL ?= openssl
 FAKETIME ?= faketime
-MBEDTLS_CERT_WRITE ?= $(PWD)/../../programs/x509/cert_write
-MBEDTLS_CERT_REQ ?= $(PWD)/../../programs/x509/cert_req
+
+TOP_DIR = $(dir $(firstword $(MAKEFILE_LIST)))/../..
+MBEDTLS_CERT_WRITE ?= $(TOP_DIR)/programs/x509/cert_write
+MBEDTLS_CERT_REQ ?= $(TOP_DIR)/programs/x509/cert_req
 
 ## Build the generated test data. Note that since the final outputs
 ## are committed to the repository, this target should do nothing on a