diff --git a/common/json.c b/common/json.c
index d1685c56f..67cc1e60e 100644
--- a/common/json.c
+++ b/common/json.c
@@ -41,6 +41,28 @@ bool json_tok_streq(const char *buffer, const jsmntok_t *tok, const char *str)
 	return strncmp(buffer + tok->start, str, tok->end - tok->start) == 0;
 }
 
+bool json_tok_startswith(const char *buffer, const jsmntok_t *tok,
+			 const char *prefix)
+{
+	if (tok->type != JSMN_STRING)
+		return false;
+	if (tok->end - tok->start < strlen(prefix))
+		return false;
+	return memcmp(buffer + tok->start,
+		      prefix, strlen(prefix)) == 0;
+}
+
+bool json_tok_endswith(const char *buffer, const jsmntok_t *tok,
+		       const char *suffix)
+{
+	if (tok->type != JSMN_STRING)
+		return false;
+	if (tok->end - tok->start < strlen(suffix))
+		return false;
+	return memcmp(buffer + tok->end - strlen(suffix),
+		      suffix, strlen(suffix)) == 0;
+}
+
 char *json_strdup(const tal_t *ctx, const char *buffer, const jsmntok_t *tok)
 {
 	return tal_strndup(ctx, buffer + tok->start, tok->end - tok->start);
diff --git a/common/json.h b/common/json.h
index c9c2f6613..19ce012a9 100644
--- a/common/json.h
+++ b/common/json.h
@@ -26,6 +26,14 @@ int json_tok_full_len(const jsmntok_t *t);
 /* Is this a string equal to str? */
 bool json_tok_streq(const char *buffer, const jsmntok_t *tok, const char *str);
 
+/* Does this string token start with prefix? */
+bool json_tok_startswith(const char *buffer, const jsmntok_t *tok,
+			 const char *prefix);
+
+/* Does this string token end with suffix? */
+bool json_tok_endswith(const char *buffer, const jsmntok_t *tok,
+		       const char *suffix);
+
 /* Allocate a tal string copy */
 char *json_strdup(const tal_t *ctx, const char *buffer, const jsmntok_t *tok);
 
diff --git a/common/json_tok.c b/common/json_tok.c
index 17d094032..2a5e5dbd4 100644
--- a/common/json_tok.c
+++ b/common/json_tok.c
@@ -313,41 +313,28 @@ struct command_result *param_feerate_val(struct command *cmd,
 					 const jsmntok_t *tok,
 					 u32 **feerate_per_kw)
 {
-	jsmntok_t base = *tok, suffix = *tok;
+	jsmntok_t base = *tok;
 	enum feerate_style style;
 	unsigned int num;
 
-	/* We have to split the number and suffix. */
-	suffix.start = suffix.end;
-	while (suffix.start > base.start && !isdigit(buffer[suffix.start-1])) {
-		suffix.start--;
-		base.end--;
-	}
+	if (json_tok_endswith(buffer, tok,
+			      feerate_style_name(FEERATE_PER_KBYTE))) {
+		style = FEERATE_PER_KBYTE;
+		base.end -= strlen(feerate_style_name(FEERATE_PER_KBYTE));
+	} else if (json_tok_endswith(buffer, tok,
+				     feerate_style_name(FEERATE_PER_KSIPA))) {
+		style = FEERATE_PER_KSIPA;
+		base.end -= strlen(feerate_style_name(FEERATE_PER_KSIPA));
+	} else
+		style = FEERATE_PER_KBYTE;
 
 	if (!json_to_number(buffer, &base, &num)) {
 		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-				    "'%s' prefix should be an integer, not '%.*s'",
+				    "'%s' should be an integer with optional perkw/perkb, not '%.*s'",
 				    name, base.end - base.start,
 				    buffer + base.start);
 	}
 
-	if (suffix.end == suffix.start
-	    || json_tok_streq(buffer, &suffix,
-			      feerate_style_name(FEERATE_PER_KBYTE))) {
-		style = FEERATE_PER_KBYTE;
-	} else if (json_tok_streq(buffer, &suffix,
-				feerate_style_name(FEERATE_PER_KSIPA))) {
-		style = FEERATE_PER_KSIPA;
-	} else {
-		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-				    "'%s' suffix should be '%s' or '%s', not '%.*s'",
-				    name,
-				    feerate_style_name(FEERATE_PER_KSIPA),
-				    feerate_style_name(FEERATE_PER_KBYTE),
-				    suffix.end - suffix.start,
-				    buffer + suffix.start);
-	}
-
 	*feerate_per_kw = tal(cmd, u32);
 	**feerate_per_kw = feerate_from_style(num, style);
 	if (**feerate_per_kw < FEERATE_FLOOR)