diff --git a/common/json.c b/common/json.c
index d4b111921..d1685c56f 100644
--- a/common/json.c
+++ b/common/json.c
@@ -325,7 +325,7 @@ JSMN Result Validation Starts
  * This part of the code performs some filtering so
  * that at least some of the invalid JSON that
  * LIBJSMN accepts, will be rejected by
- * json_parse_input.
+ * json_parse_input.  It also checks that strings are valid UTF-8.
  */
 
 /*~ These functions are used in JSMN validation.
@@ -360,7 +360,8 @@ JSMN Result Validation Starts
  */
 /* Validate a *single* datum.  */
 static const jsmntok_t *
-validate_jsmn_datum(const jsmntok_t *p,
+validate_jsmn_datum(const char *buf,
+		    const jsmntok_t *p,
 		    const jsmntok_t *end,
 		    bool *valid);
 /*~ Validate a key-value pair.
@@ -384,12 +385,14 @@ validate_jsmn_datum(const jsmntok_t *p,
  * is to reject such improper arrays and objects.
  */
 static const jsmntok_t *
-validate_jsmn_keyvalue(const jsmntok_t *p,
+validate_jsmn_keyvalue(const char *buf,
+		       const jsmntok_t *p,
 		       const jsmntok_t *end,
 		       bool *valid);
 
 static const jsmntok_t *
-validate_jsmn_datum(const jsmntok_t *p,
+validate_jsmn_datum(const char *buf,
+		    const jsmntok_t *p,
 		    const jsmntok_t *end,
 		    bool *valid)
 {
@@ -402,8 +405,11 @@ validate_jsmn_datum(const jsmntok_t *p,
 	}
 
 	switch (p->type) {
-	case JSMN_UNDEFINED:
 	case JSMN_STRING:
+		if (!utf8_check(buf + p->start, p->end - p->start))
+			*valid = false;
+		/* Fall thru */
+	case JSMN_UNDEFINED:
 	case JSMN_PRIMITIVE:
 		/* These types should not have sub-datums.  */
 		if (p->size != 0)
@@ -418,7 +424,7 @@ validate_jsmn_datum(const jsmntok_t *p,
 		++p;
 		for (i = 0; i < sz; ++i) {
 			/* Arrays should only contain standard JSON datums.  */
-			p = validate_jsmn_datum(p, end, valid);
+			p = validate_jsmn_datum(buf, p, end, valid);
 			if (!*valid)
 				break;
 		}
@@ -430,7 +436,7 @@ validate_jsmn_datum(const jsmntok_t *p,
 		++p;
 		for (i = 0; i < sz; ++i) {
 			/* Objects should only contain key-value pairs.  */
-			p = validate_jsmn_keyvalue(p, end, valid);
+			p = validate_jsmn_keyvalue(buf, p, end, valid);
 			if (!*valid)
 				break;
 		}
@@ -445,7 +451,8 @@ validate_jsmn_datum(const jsmntok_t *p,
 }
 /* Key-value pairs *must* be strings with size 1.  */
 static inline const jsmntok_t *
-validate_jsmn_keyvalue(const jsmntok_t *p,
+validate_jsmn_keyvalue(const char *buf,
+		       const jsmntok_t *p,
 		       const jsmntok_t *end,
 		       bool *valid)
 {
@@ -472,13 +479,14 @@ validate_jsmn_keyvalue(const jsmntok_t *p,
 	 * incidentally rejects that non-standard
 	 * JSON.
 	 */
-	if (p->type != JSMN_STRING || p->size != 1) {
+	if (p->type != JSMN_STRING || p->size != 1
+	    || !utf8_check(buf + p->start, p->end - p->start)) {
 		*valid = false;
 		return p;
 	}
 
 	++p;
-	return validate_jsmn_datum(p, end, valid);
+	return validate_jsmn_datum(buf, p, end, valid);
 }
 
 /** validate_jsmn_parse_output
@@ -525,12 +533,13 @@ validate_jsmn_keyvalue(const jsmntok_t *p,
  * `jsmn_parse`, false otherwise.
  */
 static bool
-validate_jsmn_parse_output(const jsmntok_t *p, const jsmntok_t *end)
+validate_jsmn_parse_output(const char *buf,
+			   const jsmntok_t *p, const jsmntok_t *end)
 {
 	bool valid = true;
 
 	while (p < end && valid)
-		p = validate_jsmn_datum(p, end, &valid);
+		p = validate_jsmn_datum(buf, p, end, &valid);
 
 	return valid;
 }
@@ -583,7 +592,7 @@ again:
 	 * element. */
 	ret = json_next(*toks) - *toks;
 
-	if (!validate_jsmn_parse_output(*toks, *toks + ret))
+	if (!validate_jsmn_parse_output(input, *toks, *toks + ret))
 		return false;
 
 	/* Cut to length and return. */
diff --git a/common/test/run-json.c b/common/test/run-json.c
index 16f961ff6..a932f9f13 100644
--- a/common/test/run-json.c
+++ b/common/test/run-json.c
@@ -1,6 +1,7 @@
 #include "../json_helpers.c"
 #include <assert.h>
 #include <ccan/mem/mem.h>
+#include <ccan/tal/str/str.h>
 #include <common/json.h>
 #include <common/utils.h>
 #include <inttypes.h>
@@ -258,6 +259,34 @@ static void test_json_delve(void)
 	assert(json_tok_streq(buf, t, "lightning-rpc"));
 }
 
+static void test_json_bad_utf8(void)
+{
+	const jsmntok_t *toks;
+	char *buf;
+
+	buf = tal_strdup(tmpctx, "{\"1\":\"one\", \"2\":\"two\", \"3\":[\"three\", {\"deeper\": 17}]}");
+	toks = json_parse_simple(tmpctx, buf, strlen(buf));
+	assert(toks);
+	assert(toks->size == 3);
+
+	assert(json_tok_streq(buf, &toks[1], "1"));
+	buf[toks[1].start] = 0xC0;
+	assert(!json_parse_simple(tmpctx, buf, strlen(buf)));
+	buf[toks[1].start] = '1';
+
+	assert(json_tok_streq(buf, &toks[2], "one"));
+	buf[toks[2].start] = 0xC0;
+	assert(!json_parse_simple(tmpctx, buf, strlen(buf)));
+	buf[toks[2].start] = 'o';
+
+	assert(json_tok_streq(buf, &toks[7], "three"));
+	buf[toks[7].start] = 0xC0;
+	assert(!json_parse_simple(tmpctx, buf, strlen(buf)));
+	buf[toks[7].start] = 't';
+
+	assert(json_parse_simple(tmpctx, buf, strlen(buf)));
+}
+
 int main(void)
 {
 	setup_locale();
@@ -267,6 +296,7 @@ int main(void)
 	test_json_tok_bitcoin_amount();
 	test_json_tok_millionths();
 	test_json_delve();
+	test_json_bad_utf8();
 	assert(!taken_any());
 	take_cleanup();
 	tal_free(tmpctx);
diff --git a/lightningd/test/run-jsonrpc.c b/lightningd/test/run-jsonrpc.c
index 7bde86ac3..57e9c9630 100644
--- a/lightningd/test/run-jsonrpc.c
+++ b/lightningd/test/run-jsonrpc.c
@@ -156,8 +156,8 @@ static int test_json_filter(void)
 	toks = toks_alloc(str);
 	jsmn_init(&parser);
 	valid = json_parse_input(&parser, &toks, str, strlen(str), &complete);
-	assert(valid);
-	assert(complete);
+	/* Fails to parse because it's not valid UTF8 */
+	assert(!valid);
 
 	assert(toks[0].type == JSMN_OBJECT);
 	x = json_get_member(str, toks, "x");