diff --git a/README.md b/README.md
index ffa1054..0d08ba3 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,7 @@ Alee Forth uses the [Forth 2012 test suite](https://github.com/gerryjackson/fort
 **Missing** core extension words:
 
 ```
-PARSE-NAME REFILL RESTORE-INPUT S\" SAVE-INPUT SOURCE-ID
+PARSE-NAME REFILL RESTORE-INPUT S\" SAVE-INPUT
 ```
 
 ## Building
diff --git a/forth/core-ext.fth b/forth/core-ext.fth
index 0c263d9..8bc83be 100644
--- a/forth/core-ext.fth
+++ b/forth/core-ext.fth
@@ -74,3 +74,4 @@
             rot dup >r c! r> char+ swap repeat
             2drop r> tuck - ;
 
+: source-id _source @ _begin < if 0 else -1 then ;