diff --git a/Makefile b/Makefile
index 84fc42201..6b22f54aa 100644
--- a/Makefile
+++ b/Makefile
@@ -33,9 +33,11 @@ CORE_NICKNAME?=	Not Yet
 CORE_NAME?=	opnsense-devel
 CORE_TYPE?=	development
 
-CORE_ABI?=	21.1
-CORE_PHP?=	74
-CORE_PYTHON?=	37
+.for REPLACEMENT in ABI PHP PYTHON
+. if empty(CORE_${REPLACEMENT})
+.  error Cannot build without CORE_${REPLACEMENT} set
+. endif
+.endfor
 
 _CORE_NEXT=	${CORE_ABI:C/\./ /}
 .if ${_CORE_NEXT:[2]} == 7