diff --git a/src/vim9.h b/src/vim9.h
index faf2650..fd98bb5 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -112,6 +112,7 @@
     // function call
     ISN_BCALL,	    // call builtin function isn_arg.bfunc
     ISN_DCALL,	    // call def function isn_arg.dfunc
+    ISN_METHODCALL, // call method on interface, uses isn_arg.mfunc
     ISN_UCALL,	    // call user function or funcref/partial isn_arg.ufunc
     ISN_PCALL,	    // call partial, use isn_arg.pfunc
     ISN_PCALL_END,  // cleanup after ISN_PCALL with cpf_top set
@@ -234,6 +235,13 @@
     int	    cdf_argcount;   // number of arguments on top of stack
 } cdfunc_T;
 
+// arguments to ISN_METHODCALL
+typedef struct {
+    class_T *cmf_itf;	    // interface used
+    int	    cmf_idx;	    // index in "def_functions" for ISN_DCALL
+    int	    cmf_argcount;   // number of arguments on top of stack
+} cmfunc_T;
+
 // arguments to ISN_PCALL
 typedef struct {
     int	    cpf_top;	    // when TRUE partial is above the arguments
@@ -517,6 +525,7 @@
 	trycont_T	    trycont;
 	cbfunc_T	    bfunc;
 	cdfunc_T	    dfunc;
+	cmfunc_T	    *mfunc;
 	cpfunc_T	    pfunc;
 	cufunc_T	    ufunc;
 	echo_T		    echo;