diff --git a/libsrc/plus4/kreadst.s b/libsrc/plus4/kreadst.s
index afebbc884..2571f80f5 100644
--- a/libsrc/plus4/kreadst.s
+++ b/libsrc/plus4/kreadst.s
@@ -7,6 +7,7 @@
         .export         READST
 
         .include        "plus4.inc"
+        .importzp       ST
 
 ; Read the status byte from the zero page instead of banking in the ROM