diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index d03503646..8b9bdba36 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -146,5 +146,5 @@ ifeq "" "$(BUILD_NUMBER)"
   # If no BUILD_NUMBER is set, create a useful "I am an engineering build
   # from this date/time" value.  Make it start with a non-digit so that
   # anyone trying to parse it as an integer will probably get "0".
-  BUILD_NUMBER := eng.$(USER).$(shell $(DATE) +%Y%m%d.%H%M%S)
+  BUILD_NUMBER := eng.$(shell echo $${USER:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
 endif