diff --git a/common/json.c b/common/json.c
index 54290cfc8..6455ece61 100644
--- a/common/json.c
+++ b/common/json.c
@@ -493,6 +493,56 @@ void json_add_string(struct json_result *result, const char *fieldname, const ch
 	result_append_fmt(result, "\"%s\"", escaped);
 }
 
+void json_add_string_escape(struct json_result *result, const char *fieldname,
+			    const char *value)
+{
+	/* Worst case: all \uXXXX */
+	char *escaped = tal_arr(result, char, strlen(value) * 6 + 1);
+	size_t i, n;
+
+	json_start_member(result, fieldname);
+	for (i = n = 0; value[i]; i++, n++) {
+		char esc = 0;
+		switch (value[i]) {
+		case '\n':
+			esc = 'n';
+			break;
+		case '\b':
+			esc = 'b';
+			break;
+		case '\f':
+			esc = 'f';
+			break;
+		case '\t':
+			esc = 't';
+			break;
+		case '\r':
+			esc = 'r';
+			break;
+		case '\\':
+		case '"':
+			esc = value[i];
+			break;
+		default:
+			if ((unsigned)value[i] < ' '
+			    || value[i] == 127) {
+				sprintf(escaped + n, "\\u%04X", value[i]);
+				n += 5;
+				continue;
+			}
+		}
+		if (esc) {
+			escaped[n++] = '\\';
+			escaped[n] = esc;
+		} else
+			escaped[n] = value[i];
+	}
+
+	escaped[n] = '\0';
+	result_append_fmt(result, "\"%s\"", escaped);
+	tal_free(escaped);
+}
+
 void json_add_bool(struct json_result *result, const char *fieldname, bool value)
 {
 	json_start_member(result, fieldname);
diff --git a/common/json.h b/common/json.h
index 8e3100824..796f387fd 100644
--- a/common/json.h
+++ b/common/json.h
@@ -83,8 +83,15 @@ void json_object_end(struct json_result *ptr);
 
 struct json_result *new_json_result(const tal_t *ctx);
 
-/* '"fieldname" : "value"' or '"value"' if fieldname is NULL*/
+/* '"fieldname" : "value"' or '"value"' if fieldname is NULL.  Turns
+ * any unusual chars into ?.
+ */
 void json_add_string(struct json_result *result, const char *fieldname, const char *value);
+
+/* Properly escapes any characters in @value */
+void json_add_string_escape(struct json_result *result, const char *fieldname,
+			    const char *value);
+
 /* '"fieldname" : literal' or 'literal' if fieldname is NULL*/
 void json_add_literal(struct json_result *result, const char *fieldname,
 		      const char *literal, int len);
diff --git a/common/test/run-json.c b/common/test/run-json.c
index 8c21ed7f1..c9bb663e5 100644
--- a/common/test/run-json.c
+++ b/common/test/run-json.c
@@ -37,8 +37,7 @@ static int test_json_tok_bitcoin_amount(void)
 	return 0;
 }
 
-
-static int test_json_escape(void)
+static int test_json_filter(void)
 {
 	struct json_result *result = new_json_result(NULL);
 	jsmntok_t *toks;
@@ -78,9 +77,41 @@ static int test_json_escape(void)
 	return 0;
 }
 
+static void test_json_escape(void)
+{
+	int i;
+	const char *str;
+
+	for (i = 1; i < 256; i++) {
+		char badstr[2];
+		struct json_result *result = new_json_result(NULL);
+
+		badstr[0] = i;
+		badstr[1] = 0;
+
+		json_object_start(result, NULL);
+		json_add_string_escape(result, "x", badstr);
+		json_object_end(result);
+
+		str = json_result_string(result);
+		if (i == '\\' || i == '"'
+		    || i == '\n' || i == '\r' || i == '\b'
+		    || i == '\t' || i == '\f')
+			assert(strstarts(str, "{ \"x\" : \"\\"));
+		else if (i < 32 || i == 127)
+			assert(strstarts(str, "{ \"x\" : \"\\u00"));
+		else {
+			char expect[] = "{ \"x\" : \"?\" }";
+			expect[9] = i;
+			assert(streq(str, expect));
+		}
+		tal_free(result);
+	}
+}
 
 int main(void)
 {
 	test_json_tok_bitcoin_amount();
+	test_json_filter();
 	test_json_escape();
 }