diff --git a/scripts/Makefile.modinst b/scripts/Makefile.modinst
index f0ff248..efa5d94 100644
--- a/scripts/Makefile.modinst
+++ b/scripts/Makefile.modinst
@@ -21,7 +21,7 @@
 
 # Modules built outside the kernel source tree go into extra by default
 INSTALL_MOD_DIR ?= extra
-ext-mod-dir = $(INSTALL_MOD_DIR)$(subst $(KBUILD_EXTMOD),,$(@D))
+ext-mod-dir = $(INSTALL_MOD_DIR)$(subst $(patsubst %/,%,$(KBUILD_EXTMOD)),,$(@D))
 
 modinst_dir = $(if $(KBUILD_EXTMOD),$(ext-mod-dir),kernel/$(@D))