diff --git a/mk/docs.mk b/mk/docs.mk
index 3f8b677185f..8af47700f25 100644
--- a/mk/docs.mk
+++ b/mk/docs.mk
@@ -283,6 +283,6 @@ compiler-docs: $(COMPILER_DOC_TARGETS)
 
 trpl: doc/book/index.html
 
-doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/trpl/*.md)
+doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/trpl/*.md) | doc/
 	$(Q)rm -rf doc/book
 	$(Q)$(RUSTBOOK) build $(S)src/doc/trpl doc/book