diff --git a/build/Makefile.in b/build/Makefile.in
index 0b592470628..f767c12cbe8 100644
--- a/build/Makefile.in
+++ b/build/Makefile.in
@@ -84,7 +84,7 @@ DEFINES += -DGRE_MILESTONE=$(GRE_MILESTONE) -DAPP_BUILDID=$(APP_BUILDID)
 DEFINES += -DMOZ_APP_VERSION="$(MOZ_APP_VERSION)"
 APP_INI_DEPS += $(DEPTH)/config/autoconf.mk
 
-MOZ_SOURCE_STAMP ?= $(firstword $(shell hg -R $(topsrcdir)/$(MOZ_BUILD_APP)/.. parent --template="{node|short}\n" 2>/dev/null))
+MOZ_SOURCE_STAMP := $(firstword $(shell hg -R $(topsrcdir)/$(MOZ_BUILD_APP)/.. parent --template="{node|short}\n" 2>/dev/null))
 ifdef MOZ_SOURCE_STAMP
 DEFINES += -DMOZ_SOURCE_STAMP="$(MOZ_SOURCE_STAMP)"
 endif