diff --git a/common/json_parse.c b/common/json_parse.c
index 5c1c68310b58..570f15e7b6a7 100644
--- a/common/json_parse.c
+++ b/common/json_parse.c
@@ -507,29 +507,21 @@ const char *json_scan(const tal_t *ctx,
 bool json_to_bitcoin_amount(const char *buffer, const jsmntok_t *tok,
 			    uint64_t *satoshi)
 {
-	char *end;
-	unsigned long btc, sat;
+	const char *start = buffer + tok->start;
+	size_t len = tok->end - tok->start;
+	const char *dot = memchr(start, '.', len);
+	u64 btc, sat = 0;
 
-	errno = 0;
-	btc = strtoul(buffer + tok->start, &end, 10);
-	if (btc == ULONG_MAX && errno == ERANGE)
-		return false;
-	if (end != buffer + tok->end) {
-		/* Expect always 8 decimal places. */
-		if (*end != '.' || buffer + tok->end - end != 9)
-			return false;
-		sat = strtoul(end+1, &end, 10);
-		if (sat == ULONG_MAX && errno == ERANGE)
-			return false;
-		if (end != buffer + tok->end)
+	if (dot) {
+		if (start + len - (dot + 1) != 8 || !str_to_u64(dot + 1, 8, &sat))
 			return false;
-	} else
-		sat = 0;
-
-	*satoshi = btc * (uint64_t)100000000 + sat;
-	if (*satoshi != btc * (uint64_t)100000000 + sat)
+		len = dot - start;
+	}
+	if (!str_to_u64(start, len, &btc))
 		return false;
-
+	if (mul_overflows_u64(btc, 100000000) || add_overflows_u64(btc * 100000000, sat))
+		return false;
+	*satoshi = btc * 100000000 + sat;
 	return true;
 }
 
diff --git a/common/test/run-json.c b/common/test/run-json.c
index 1690972b8eb0..bc44d2c50a6e 100644
--- a/common/test/run-json.c
+++ b/common/test/run-json.c
@@ -51,7 +51,7 @@ void towire_tlv(u8 **pptr UNNEEDED,
 
 // issue #577
 
-static void do_json_tok_bitcoin_amount(const char* val, uint64_t expected)
+static void do_json_tok_bitcoin_amount(const char *val, bool ok, uint64_t expected)
 {
 	uint64_t amount;
 	jsmntok_t tok;
@@ -59,10 +59,12 @@ static void do_json_tok_bitcoin_amount(const char* val, uint64_t expected)
 	tok.start = 0;
 	tok.end = strlen(val);
 
-	fprintf(stderr, "do_json_tok_bitcoin_amount(\"%s\", %"PRIu64"): ", val, expected);
+	fprintf(stderr, "do_json_tok_bitcoin_amount(\"%s\", %s, %"PRIu64"): ",
+		val, ok ? "true" : "false", expected);
 
-	assert(json_to_bitcoin_amount(val, &tok, &amount) == true);
-	assert(amount == expected);
+	assert(json_to_bitcoin_amount(val, &tok, &amount) == ok);
+	if (ok)
+		assert(amount == expected);
 
 	fprintf(stderr, "ok\n");
 }
@@ -70,13 +72,31 @@ static void do_json_tok_bitcoin_amount(const char* val, uint64_t expected)
 
 static int test_json_tok_bitcoin_amount(void)
 {
-	do_json_tok_bitcoin_amount("0.00000001", 1);
-	do_json_tok_bitcoin_amount("0.00000007", 7);
-	do_json_tok_bitcoin_amount("0.00000008", 8);
-	do_json_tok_bitcoin_amount("0.00000010", 10);
-	do_json_tok_bitcoin_amount("0.12345678", 12345678);
-	do_json_tok_bitcoin_amount("0.01234567", 1234567);
-	do_json_tok_bitcoin_amount("123.45678900", 12345678900);
+	do_json_tok_bitcoin_amount("0", true, 0);
+	do_json_tok_bitcoin_amount("0.00000000", true, 0);
+	do_json_tok_bitcoin_amount("0.00000001", true, 1);
+	do_json_tok_bitcoin_amount("0.00000007", true, 7);
+	do_json_tok_bitcoin_amount("0.00000008", true, 8);
+	do_json_tok_bitcoin_amount("0.00000010", true, 10);
+	do_json_tok_bitcoin_amount("0.12345678", true, 12345678);
+	do_json_tok_bitcoin_amount("0.01234567", true, 1234567);
+	do_json_tok_bitcoin_amount("123.45678900", true, 12345678900);
+	do_json_tok_bitcoin_amount("21000000.00000000", true, 2100000000000000ULL);
+
+	do_json_tok_bitcoin_amount("184467440737.09551615", true, UINT64_MAX);
+
+	/* Invalid inputs */
+	do_json_tok_bitcoin_amount("", false, 0);
+	do_json_tok_bitcoin_amount("-1", false, 0);
+	do_json_tok_bitcoin_amount("-0.00000001", false, 0);
+	do_json_tok_bitcoin_amount("+1.00000000", false, 0);
+	do_json_tok_bitcoin_amount("1.0000000", false, 0);  /* 7 decimals */
+	do_json_tok_bitcoin_amount("1.000000000", false, 0); /* 9 decimals */
+	do_json_tok_bitcoin_amount("184467440738.00000000", false, 0); /* overflows u64 */
+	do_json_tok_bitcoin_amount("184467440737.09551616", false, 0); /* add overflows */
+	do_json_tok_bitcoin_amount("1.+0000001", false, 0);
+	do_json_tok_bitcoin_amount("1.-0000001", false, 0);
+	do_json_tok_bitcoin_amount(" 1.00000000", false, 0);
 
 	return 0;
 }