diff --git a/core/main.mk b/core/main.mk
index 99e4e43..cb44106 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1341,7 +1341,7 @@
 endif
 
 # Build docs as part of checkbuild to catch more breakages.
-module_to_check += $(ALL_DOCS)
+modules_to_check += $(ALL_DOCS)
 
 # for easier debugging
 modules_to_check := $(sort $(modules_to_check))