diff --git a/src/Makefile_obj.in b/src/Makefile_obj.in
index 8313ba6f1..8d63aeac7 100644
--- a/src/Makefile_obj.in
+++ b/src/Makefile_obj.in
@@ -288,7 +288,7 @@ V3Number_test: V3Number_test.o
 #### Modules
 
 %__gen.cpp:	%.cpp $(ASTGEN) V3Ast.h V3AstNodes.h
-	$(PYTHON3) $(ASTGEN) -I$(srcdir) $*.cpp
+	$(PYTHON3) $(ASTGEN) -I $(srcdir) $*.cpp
 
 %.o:	%.cpp
 	$(OBJCACHE) ${CXX} ${CXXFLAGS} ${CPPFLAGSWALL} -c $< -o $@