diff --git a/ports/zsh/Makefile b/ports/zsh/Makefile
index bb66da6..8e799be 100644
--- a/ports/zsh/Makefile
+++ b/ports/zsh/Makefile
@@ -195,7 +195,15 @@ licence-check: | $(SRCDIR)
 	if [ $$bad -ne 0 ]; then echo "zsh: refusing to install" >&2; exit 1; fi; \
 	echo "zsh: no GPL-licensed files in the tree"
 
-$(SYSROOT)/bin/zsh: $(BUILDDIR)/Src/zsh licence-check | $(SYSROOT)/bin
+# licence-check runs *inside* the rule, not as a prerequisite.
+#
+# It is .PHONY, and a phony prerequisite is always out of date -- so having it
+# on the left of the colon re-ran the whole install on every `make`, which is
+# slow, and which eventually failed outright when zsh's installfns.sh tripped
+# over the installfnsdir its previous run had left behind.
+$(SYSROOT)/bin/zsh: $(BUILDDIR)/Src/zsh | $(SYSROOT)/bin
+	$(MAKE) licence-check
+	rm -rf $(BUILDDIR)/installfnsdir
 	$(MAKE) -C $(BUILDDIR) install DESTDIR=$(SYSROOT)
 	rm -rf $(SYSROOT)/usr/share/man $(SYSROOT)/usr/share/zsh/$(ZSH_VERSION)/help
 	chmod 755 $@