diff --git a/py/makeqstrdefs.py b/py/makeqstrdefs.py
index ad4f22d5e4..187a9aeeaa 100644
--- a/py/makeqstrdefs.py
+++ b/py/makeqstrdefs.py
@@ -34,7 +34,7 @@ def preprocess():
     for source in sources:
         if source.endswith(".cpp"):
             cxxsources.append(source)
-        else:
+        elif source.endswith(".c"):
             csources.append(source)
     try:
         os.makedirs(os.path.dirname(args.output[0]))
diff --git a/py/mkrules.mk b/py/mkrules.mk
index eb8477cbb1..b8d07ef56d 100644
--- a/py/mkrules.mk
+++ b/py/mkrules.mk
@@ -100,7 +100,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h $(OBJ
 # - else, if list of newer prerequisites ($?) is not empty, then process just these ($?)
 # - else, process all source files ($^) [this covers "make -B" which can set $? to empty]
 # See more information about this process in docs/develop/qstr.rst.
-$(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(QSTR_GLOBAL_REQUIREMENTS)
+$(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) $(HEADER_BUILD)/moduledefs.h | $(QSTR_GLOBAL_REQUIREMENTS)
 	$(ECHO) "GEN $@"
 	$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py pp $(CPP) output $(HEADER_BUILD)/qstr.i.last cflags $(QSTR_GEN_CFLAGS) cxxflags $(QSTR_GEN_CXXFLAGS) sources $^ dependencies $(QSTR_GLOBAL_DEPENDENCIES) changed_sources $?
 
diff --git a/py/py.mk b/py/py.mk
index 59abc8f503..ab01b0ec21 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -269,8 +269,6 @@ $(HEADER_BUILD)/moduledefs.h: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER
 	@$(ECHO) "GEN $@"
 	$(Q)$(PYTHON) $(PY_SRC)/makemoduledefs.py --vpath="., $(TOP), $(USER_C_MODULES)" $(SRC_QSTR) > $@
 
-SRC_QSTR += $(HEADER_BUILD)/moduledefs.h
-
 # Standard C functions like memset need to be compiled with special flags so
 # the compiler does not optimise these functions in terms of themselves.
 CFLAGS_BUILTIN ?= -ffreestanding -fno-builtin -fno-lto