--- a/make/Bundles.gmk	Tue Feb 05 15:00:10 2019 +0100
+++ b/make/Bundles.gmk	Tue Feb 05 16:27:08 2019 +0100
@@ -35,7 +35,7 @@
 # On Windows tar frequently complains that "file changed as we read it" for
 # some random source files. This seems to be cause by anti virus scanners and
 # is most likely safe to ignore. When it happens, tar returns '1'.
-ifeq ($(isBuildOs, windows), true)
+ifeq ($(call isBuildOs, windows), true)
   TAR_IGNORE_EXIT_VALUE := || test "$$$$?" = "1"
 endif