diff --git a/Makefile b/Makefile
index 9b4fb23..5f842a1 100644
--- a/Makefile
+++ b/Makefile
@@ -275,7 +275,7 @@ $(GEN)/pico_ice_boot.h: tools/source_to_string.js $(GEN)/pico_ice_boot_merged.fs
 	$< boot $(VERSION) $(REVISION) $(GEN)/pico_ice_boot_merged.fs >$@
 
 $(GEN)/esp32_boot_merged.fs: esp32/esp32_boot.fs | $(GEN)
-	./tools/importation.py $^ $@ \
+	./tools/importation.py $< $@ \
     -I . -I $(GEN) --depsout $@.dd \
     --set-version $(VERSION) \
     --set-revision $(REVISION)
diff --git a/tools/importation.py b/tools/importation.py
index e7dfe3d..03ad8de 100755
--- a/tools/importation.py
+++ b/tools/importation.py
@@ -36,6 +36,7 @@ def Import(filename):
       elif (filename.endswith('.h') or
             filename.endswith('.ino') or
             filename.endswith('.cc') or
+            filename.endswith('.cpp') or
             filename.endswith('.c')) and line.startswith('#include "'):
         sfilename = line.split('"')[1]
         done = False