diff --git a/gitstatus/Makefile b/gitstatus/Makefile
index a6e02a02..0141dd90 100644
--- a/gitstatus/Makefile
+++ b/gitstatus/Makefile
@@ -2,7 +2,7 @@ APPNAME ?= gitstatusd
 OBJDIR ?= obj
 
 CXX ?= g++
-ZSH ?= $(shell command -v zsh 2> /dev/null)
+ZSH := $(shell command -v zsh 2> /dev/null)
 
 VERSION ?= $(shell . ./build.info && printf "%s" "$$gitstatus_version")