diff --git a/src/Makefile_obj.in b/src/Makefile_obj.in
index 8d63aeac7..9fbc08295 100644
--- a/src/Makefile_obj.in
+++ b/src/Makefile_obj.in
@@ -319,7 +319,7 @@ vlcovgen.d: $(VLCOVGEN) $(srcdir)/../include/verilated_cov_key.h
 	touch $@
 
 V3Ast__gen_classes.h : $(ASTGEN) V3Ast.h V3AstNodes.h
-	$(PYTHON3) $(ASTGEN) -I$(srcdir)  --classes
+	$(PYTHON3) $(ASTGEN) -I $(srcdir)  --classes
 
 V3ParseBison.h: V3ParseBison.c