diff --git a/src/strings.c b/src/strings.c
index 98d420c..bc0fc5e 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -904,8 +904,7 @@
 
     if (in_vim9script()
 	    && (check_for_string_arg(argvars, 0) == FAIL
-		|| (argvars[1].v_type != VAR_UNKNOWN
-		    && check_for_bool_arg(argvars, 1) == FAIL)))
+		|| check_for_opt_bool_arg(argvars, 1) == FAIL))
 	return;
 
     if (argvars[1].v_type != VAR_UNKNOWN)
@@ -1116,8 +1115,7 @@
 
     if (in_vim9script()
 	    && (check_for_string_arg(argvars, 0) == FAIL
-		|| (argvars[1].v_type != VAR_UNKNOWN
-		    && check_for_bool_arg(argvars, 1) == FAIL)))
+		|| check_for_opt_bool_arg(argvars, 1) == FAIL))
 	return;
 
     if (argvars[1].v_type != VAR_UNKNOWN)
@@ -1141,8 +1139,7 @@
 
     if (in_vim9script()
 	    && (check_for_string_arg(argvars, 0) == FAIL
-		|| (argvars[1].v_type != VAR_UNKNOWN
-		    && check_for_number_arg(argvars, 1) == FAIL)))
+		|| check_for_opt_number_arg(argvars, 1) == FAIL))
 	return;
 
     s = tv_get_string(&argvars[0]);
@@ -1178,6 +1175,14 @@
     int		slen;
     int		error = FALSE;
 
+    if (in_vim9script()
+	    && (check_for_string_arg(argvars, 0) == FAIL
+		|| check_for_number_arg(argvars, 1) == FAIL
+		|| check_for_opt_number_arg(argvars, 2) == FAIL
+		|| (argvars[2].v_type != VAR_UNKNOWN
+		    && check_for_opt_bool_arg(argvars, 3) == FAIL)))
+	return;
+
     p = tv_get_string(&argvars[0]);
     slen = (int)STRLEN(p);
 
@@ -1261,6 +1266,14 @@
     int		slen;
     int		error = FALSE;
 
+    if (in_vim9script()
+	    && (check_for_string_arg(argvars, 0) == FAIL
+		|| check_for_number_arg(argvars, 1) == FAIL
+		|| check_for_opt_number_arg(argvars, 2) == FAIL
+		|| (argvars[2].v_type != VAR_UNKNOWN
+		    && check_for_opt_bool_arg(argvars, 3) == FAIL)))
+	return;
+
     p = tv_get_string(&argvars[0]);
     slen = (int)STRLEN(p);