diff --git a/Makefile b/Makefile
index 1785904a..c3f22568 100644
--- a/Makefile
+++ b/Makefile
@@ -89,7 +89,7 @@ ABCMKARGS = CC="$(CXX)" CXX="$(CXX)"
 
 # set ABCEXTERNAL = <abc-command> to use an external ABC instance
 # Note: The in-tree ABC (yosys-abc) will not be installed when ABCEXTERNAL is set.
-ABCEXTERNAL =
+ABCEXTERNAL ?=
 
 define newline