diff --git a/Makefile b/Makefile
index 8560b79..c319144 100644
--- a/Makefile
+++ b/Makefile
@@ -1193,6 +1193,17 @@
 __srctree = $(srctree)/
 endif
 
+ifeq ($(ALLSOURCE_ARCHS),)
+ifeq ($(ARCH),um)
+ALLINCLUDE_ARCHS := $(ARCH) $(SUBARCH)
+else
+ALLINCLUDE_ARCHS := $(ARCH)
+endif
+else
+#Allow user to specify only ALLSOURCE_PATHS on the command line, keeping existing behaviour.
+ALLINCLUDE_ARCHS := $(ALLSOURCE_ARCHS)
+endif
+
 ALLSOURCE_ARCHS := $(ARCH)
 
 define all-sources
@@ -1208,7 +1219,7 @@
 	  find $(__srctree)include $(RCS_FIND_IGNORE) \
 	       \( -name config -o -name 'asm-*' \) -prune \
 	       -o -name '*.[chS]' -print; \
-	  for ARCH in $(ALLSOURCE_ARCHS) ; do \
+	  for ARCH in $(ALLINCLUDE_ARCHS) ; do \
 	       find $(__srctree)include/asm-$${ARCH} $(RCS_FIND_IGNORE) \
 	            -name '*.[chS]' -print; \
 	  done ; \