diff --git a/Makefile b/Makefile
index 3e134ec..f4702c1 100644
--- a/Makefile
+++ b/Makefile
@@ -149,7 +149,17 @@
 _all: modules
 endif
 
-srctree		:= $(if $(KBUILD_SRC),$(KBUILD_SRC),.)
+ifeq ($(KBUILD_SRC),)
+        # building in the source tree
+        srctree := .
+else
+        ifeq ($(KBUILD_SRC)/,$(dir $(CURDIR)))
+                # building in a subdirectory of the source tree
+                srctree := ..
+        else
+                srctree := $(KBUILD_SRC)
+        endif
+endif
 objtree		:= .
 src		:= $(srctree)
 obj		:= $(objtree)