diff --git a/src/makefiles/pgxs.mk b/src/makefiles/pgxs.mk
index ea17b1c1858624d196776cf2a65913d4776137b7..08e25da72319198550611f7ad151b8812c68e20e 100644
--- a/src/makefiles/pgxs.mk
+++ b/src/makefiles/pgxs.mk
@@ -294,7 +294,7 @@ else
 check: all submake $(REGRESS_PREP)
 	$(pg_regress_check) $(REGRESS_OPTS) $(REGRESS)
 
-temp-install: EXTRA_INSTALL=$(subdir)
+temp-install: EXTRA_INSTALL+=$(subdir)
 endif
 endif # REGRESS