diff --git a/solenv/gbuild/LinkTarget.mk b/solenv/gbuild/LinkTarget.mk
index 69683c5..d003455 100644
--- a/solenv/gbuild/LinkTarget.mk
+++ b/solenv/gbuild/LinkTarget.mk
@@ -1069,9 +1069,9 @@ endef
 # this forwards to functions that must be defined in RepositoryExternal.mk.
 # $(eval $(call gb_LinkTarget_use_external,library,external))
 define gb_LinkTarget_use_external
-$(if $(value gb_LinkTarget__use_$(2)),\
-  $(call gb_LinkTarget__use_$(2),$(1)),\
-  $(error gb_LinkTarget_use_external: unknown external: $(2)))
+$(if $(filter undefined,$(origin gb_LinkTarget__use_$(2))),\
+  $(error gb_LinkTarget_use_external: unknown external: $(2)),\
+  $(call gb_LinkTarget__use_$(2),$(1)))
 endef
 
 # $(call gb_LinkTarget_use_externals,library,externals)