diff --git a/src/vim9.h b/src/vim9.h
index 58d451c..97b9a3e 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -168,8 +168,7 @@
     ISN_PROF_START, // start a line for profiling
     ISN_PROF_END,   // end a line for profiling
 
-    ISN_DEBUG,	    // check for debug breakpoint, isn_arg.number is current
-		    // number of local variables
+    ISN_DEBUG,	    // check for debug breakpoint, uses isn_arg.debug
 
     ISN_UNPACK,	    // unpack list into items, uses isn_arg.unpack
     ISN_SHUFFLE,    // move item on stack up or down
@@ -391,6 +390,12 @@
     int		invert;
 } tobool_T;
 
+// arguments to ISN_DEBUG
+typedef struct {
+    varnumber_T	dbg_var_names_len;  // current number of local variables
+    int		dbg_break_lnum;	    // first line to break after
+} debug_T;
+
 /*
  * Instruction
  */
@@ -439,6 +444,7 @@
 	tostring_T	    tostring;
 	tobool_T	    tobool;
 	getitem_T	    getitem;
+	debug_T		    debug;
     } isn_arg;
 };