diff --git a/src/test/isolation/Makefile b/src/test/isolation/Makefile
index 1d4c7db8bc4..ae04f6cc1a2 100644
--- a/src/test/isolation/Makefile
+++ b/src/test/isolation/Makefile
@@ -10,7 +10,7 @@ ifeq ($(PORTNAME), win32)
 LDLIBS += -lws2_32
 endif
 
-override CPPFLAGS := -I$(libpq_srcdir) $(CPPFLAGS)
+override CPPFLAGS := -I$(srcdir) -I$(libpq_srcdir) $(CPPFLAGS)
 override LDLIBS := $(libpq_pgport) $(LDLIBS)
 
 OBJS =  specparse.o isolationtester.o