diff --git a/src/test/isolation/Makefile b/src/test/isolation/Makefile
index ab80409e12c..2a6132ca3b1 100644
--- a/src/test/isolation/Makefile
+++ b/src/test/isolation/Makefile
@@ -9,7 +9,8 @@ subdir = src/test/isolation
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
-override CPPFLAGS := -I$(srcdir) -I$(libpq_srcdir) -I$(srcdir)/../regress $(CPPFLAGS)
+override CPPFLAGS := -I. -I$(srcdir) -I$(libpq_srcdir) \
+	-I$(srcdir)/../regress $(CPPFLAGS)
 
 OBJS =  specparse.o isolationtester.o $(WIN32RES)