diff --git a/src/pdu.c b/src/pdu.c
index 741c3fe6..8cec5266 100644
--- a/src/pdu.c
+++ b/src/pdu.c
@@ -1014,7 +1014,7 @@ coap_pdu_parse_opt_base(coap_pdu_t *pdu, uint16_t len) {
 static int
 write_prefix(char **obp, size_t *len, const char *prf, size_t prflen) {
   /* Make sure space for null terminating byte */
-  if (*len > prflen +1) {
+  if (*len < prflen +1) {
     return 0;
   }
 
@@ -1027,7 +1027,7 @@ write_prefix(char **obp, size_t *len, const char *prf, size_t prflen) {
 static int
 write_char(char **obp, size_t *len, char c, int printable) {
   /* Make sure space for null terminating byte */
-  if (*len > 3) {
+  if (*len < 2 +1) {
     return 0;
   }