--- a/client.mk
+++ b/client.mk
@@ -137,39 +137,35 @@ MOZCONFIG_MODULES := build/unix/uniq.pl
 
 
 
 run_for_side_effects := \
   $(shell $(TOPSRCDIR)/$(MOZCONFIG_LOADER) $(TOPSRCDIR) $(TOPSRCDIR)/.mozconfig.mk > $(TOPSRCDIR)/.mozconfig.out)
 
 include $(TOPSRCDIR)/.mozconfig.mk
 
+ifndef MOZ_OBJDIR
+  MOZ_OBJDIR = obj-$(CONFIG_GUESS)
+endif
+
 ifdef MOZ_BUILD_PROJECTS
 
-ifndef MOZ_OBJDIR
-  $(error When MOZ_BUILD_PROJECTS is set, you must set MOZ_OBJDIR)
-endif
 ifdef MOZ_CURRENT_PROJECT
   OBJDIR = $(MOZ_OBJDIR)/$(MOZ_CURRENT_PROJECT)
   MOZ_MAKE = $(MAKE) $(MOZ_MAKE_FLAGS) -C $(OBJDIR)
   BUILD_PROJECT_ARG = MOZ_BUILD_APP=$(MOZ_CURRENT_PROJECT)
 else
   OBJDIR = $(error Cannot find the OBJDIR when MOZ_CURRENT_PROJECT is not set.)
   MOZ_MAKE = $(error Cannot build in the OBJDIR when MOZ_CURRENT_PROJECT is not set.)
 endif
 
 else # MOZ_BUILD_PROJECTS
 
-ifdef MOZ_OBJDIR
-  OBJDIR = $(MOZ_OBJDIR)
-  MOZ_MAKE = $(MAKE) $(MOZ_MAKE_FLAGS) -C $(OBJDIR)
-else
-  OBJDIR := $(TOPSRCDIR)
-  MOZ_MAKE := $(MAKE) $(MOZ_MAKE_FLAGS)
-endif
+OBJDIR = $(MOZ_OBJDIR)
+MOZ_MAKE = $(MAKE) $(MOZ_MAKE_FLAGS) -C $(OBJDIR)
 
 endif # MOZ_BUILD_PROJECTS
 
 # 'configure' scripts generated by autoconf.
 CONFIGURES := $(TOPSRCDIR)/configure
 CONFIGURES += $(TOPSRCDIR)/js/src/configure
 
 #######################################################################