diff --git a/dtool/src/test_interrogate/Sources.pp b/dtool/src/test_interrogate/Sources.pp
index 1db502b1ef..a36d5868f3 100644
--- a/dtool/src/test_interrogate/Sources.pp
+++ b/dtool/src/test_interrogate/Sources.pp
@@ -1,3 +1,4 @@
+#define BUILD_DIRECTORY $[HAVE_INTERROGATE]
 #define LOCAL_LIBS interrogatedb dconfig dtoolutil dtoolbase pystub
 #define USE_PACKAGES openssl