diff --git a/Makefile b/Makefile
index 17e2d58ce5..b522b10195 100644
--- a/Makefile
+++ b/Makefile
@@ -8,6 +8,17 @@ ifneq ($(wildcard config-host.mak),)
 # Put the all: rule here so that config-host.mak can contain dependencies.
 all:
 include config-host.mak
+
+# Check that we're not trying to do an out-of-tree build from
+# a tree that's been used for an in-tree build.
+ifneq ($(realpath $(SRC_PATH)),$(realpath .))
+ifneq ($(wildcard $(SRC_PATH)/config-host.mak),)
+$(error This is an out of tree build but your source tree ($(SRC_PATH)) \
+seems to have been used for an in-tree build. You can fix this by running \
+"make distclean && rm -rf *-linux-user *-softmmu" in your source tree)
+endif
+endif
+
 include $(SRC_PATH)/rules.mak
 config-host.mak: $(SRC_PATH)/configure
 	@echo $@ is out-of-date, running configure