diff --git a/tools/prelude/Makefile b/tools/prelude/Makefile
index bedd021c4321cc18accee3e2459349dac12880f5..bc89ebe8eaeba5beda12aa2728c4964587dea12b 100644
--- a/tools/prelude/Makefile
+++ b/tools/prelude/Makefile
@@ -2,7 +2,7 @@ PVS2DK = ../pvs2dk.sh
 LPC    = lambdapi check
 
 THEORIES != grep -v -E '^-|\#' theories
-PVSPATH   =
+PVSPATH  ?=
 
 all: dummy ${THEORIES:C/$/.lpo/}