diff --git a/src/makefiles/pgxs.mk b/src/makefiles/pgxs.mk
index c038836e73..a97ad2e0f5 100644
--- a/src/makefiles/pgxs.mk
+++ b/src/makefiles/pgxs.mk
@@ -60,6 +60,8 @@ $(error pgxs error: makefile variable PGXS or NO_PGXS must be set)
 endif
 endif
 
+# External extensions can't build the headers
+NO_GENERATED_HEADERS=yes
 
 ifdef PGXS
 # We assume that we are in src/makefiles/, so top is ...