diff --git a/src/jtag/tcl.c b/src/jtag/tcl.c
index ceea19303..dd2be1e5a 100644
--- a/src/jtag/tcl.c
+++ b/src/jtag/tcl.c
@@ -169,7 +169,10 @@ static int Jim_Command_drscan(Jim_Interp *interp, int argc, Jim_Obj *const *args
 		return JIM_ERR;
 
 	num_fields = (argc-2)/2;
-	assert(num_fields > 0);
+	if (num_fields <= 0) {
+		Jim_SetResultString(interp, "drscan: no scan fields supplied", -1);
+		return JIM_ERR;
+	}
 	fields = malloc(sizeof(struct scan_field) * num_fields);
 	for (i = 2; i < argc; i += 2) {
 		long bits;