diff --git a/src/Makefile b/src/Makefile
index 3410d214c884c85c60ae9dc80aa77620bbdbb0e0..e888ef9d1de2a1dd6c0830acb9548d16030c488e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -39,7 +39,7 @@ endif
 
 ifeq ($(SYSMNGR_VENDOR_EXTENSIONS),y)
 PROG_CFLAGS += -DSYSMNGR_VENDOR_EXTENSIONS
-ifndef CUSTOM_PREFIX
+ifeq ($(CUSTOM_PREFIX),)
 PROG_CFLAGS += -DCUSTOM_PREFIX=\"X_IOPSYS_EU_\"
 endif
 endif