diff --git a/App/tools/driver.makefile b/App/tools/driver.makefile
index 02ef022..1780742 100644
--- a/App/tools/driver.makefile
+++ b/App/tools/driver.makefile
@@ -100,6 +100,10 @@ DEPFILE = ${PRJ}.dep
 BASH_ENV=
 ENV=
 
+ifeq (${EPICS_HOST_ARCH},)
+$(error EPICS_HOST_ARCH is not set)
+endif
+
 # Default target is "build" for all versions.
 # Don't install anything (different from default EPICS make rules).
 default: build