diff --git a/firmware/Makefile b/firmware/Makefile
index 020e629..99955ed 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -142,7 +142,7 @@
 fw-shipped-all := $(fw-shipped-y) $(fw-shipped-m) $(fw-shipped-)
 
 # Directories which we _might_ need to create, so we have a rule for them.
-firmware-dirs := $(sort $(patsubst %,$(objtree)/$(obj)/%/,$(dir $(fw-external-y) $(fw-shipped-all))))
+firmware-dirs := $(sort $(addprefix $(objtree)/$(obj)/,$(dir $(fw-external-y) $(fw-shipped-all))))
 
 quiet_cmd_mkdir = MKDIR   $(patsubst $(objtree)/%,%,$@)
       cmd_mkdir = mkdir -p $@