diff --git a/tests/libtest/Makefile.inc b/tests/libtest/Makefile.inc
index 05a81d3c0d..2a8dfc2168 100644
--- a/tests/libtest/Makefile.inc
+++ b/tests/libtest/Makefile.inc
@@ -370,7 +370,7 @@ lib1520_SOURCES = lib1520.c $(SUPPORTFILES)
 lib1520_CPPFLAGS = $(AM_CPPFLAGS) -DLIB1520
 
 lib1521_SOURCES = lib1521.c $(SUPPORTFILES)
-lib1521_CPPFLAGS = $(AM_CPPFLAGS)
+lib1521_CPPFLAGS = $(AM_CPPFLAGS) -I$(srcdir)
 
 lib1525_SOURCES = lib1525.c $(SUPPORTFILES) $(TESTUTIL) $(WARNLESS)
 lib1525_LDADD = $(TESTUTIL_LIBS)