diff --git a/Makefile b/Makefile
index 7f528cb..03c058d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,5 @@
+# RISCV should either be unset, or set to point to a directory that contains
+# a toolchain install tree that was built via other means.
 RISCV ?= $(CURDIR)/toolchain
 PATH := $(RISCV)/bin:$(PATH)
 ISA ?= rv64imafdc
@@ -59,6 +61,11 @@ all: $(hex)
 	@echo Program it with: dd if=work/bbl.bin of=/dev/sd-your-card bs=1M
 	@echo
 
+ifneq ($(RISCV),$(toolchain_dest))
+$(RISCV)/bin/$(target)-gcc:
+	$(error The RISCV environment variable was set, but is not pointing at a toolchain install tree)
+endif
+
 $(toolchain_dest)/bin/$(target)-gcc: $(toolchain_srcdir)
 	mkdir -p $(toolchain_wrkdir)
 	$(MAKE) -C $(linux_srcdir) O=$(dir $<) ARCH=riscv INSTALL_HDR_PATH=$(abspath $(toolchain_srcdir)/linux-headers) headers_install