diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c b/tools/ocaml/libs/xc/xenctrl_stubs.c
index 7bcb87e1fa..53ca4b1cef 100644
--- a/tools/ocaml/libs/xc/xenctrl_stubs.c
+++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
@@ -51,21 +51,22 @@
 	i1 = (uint32_t) Int64_val(Field(input, 0)); \
 	i2 = ((Field(input, 1) == Val_none) ? 0xffffffff : (uint32_t) Int64_val(Field(Field(input, 1), 0)));
 
-#define ERROR_STRLEN 1024
-void failwith_xc(xc_interface *xch)
+static void Noreturn failwith_xc(xc_interface *xch)
 {
-	static char error_str[ERROR_STRLEN];
+	char error_str[256];
 	if (xch) {
 		const xc_error *error = xc_get_last_error(xch);
 		if (error->code == XC_ERROR_NONE)
-                	snprintf(error_str, ERROR_STRLEN, "%d: %s", errno, strerror(errno));
+			snprintf(error_str, sizeof(error_str),
+				 "%d: %s", errno, strerror(errno));
 		else
-			snprintf(error_str, ERROR_STRLEN, "%d: %s: %s",
-				 error->code,
+			snprintf(error_str, sizeof(error_str),
+				 "%d: %s: %s", error->code,
 				 xc_error_code_to_desc(error->code),
 				 error->message);
 	} else {
-		snprintf(error_str, ERROR_STRLEN, "Unable to open XC interface");
+		snprintf(error_str, sizeof(error_str),
+			 "Unable to open XC interface");
 	}
 	caml_raise_with_string(*caml_named_value("xc.error"), error_str);
 }