diff --git a/include/mosquitto.h b/include/mosquitto.h
index 6c785bbb..840e9da5 100644
--- a/include/mosquitto.h
+++ b/include/mosquitto.h
@@ -1597,7 +1597,10 @@ libmosq_EXPORT int mosquitto_opts_set(struct mosquitto *mosq, enum mosq_opt_t op
  *
  *	MOSQ_OPT_HTTP_HEADER_SIZE - Size the size of buffer that will be allocated
  *	          to store the incoming HTTP header when using Websocket transport.
- *	          Defaults to 4096.
+ *	          Defaults to 4096. Setting to below 100 will result in a return
+*	          value of MOSQ_ERR_INVAL. This should be set before starting the
+*	          connection. If you try to set this when the initial http request
+*	          is underway then it will return MOSQ_ERR_INVAL.
  */
 libmosq_EXPORT int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t option, int value);
 
diff --git a/lib/http_client.c b/lib/http_client.c
index f563eff3..69079924 100644
--- a/lib/http_client.c
+++ b/lib/http_client.c
@@ -51,7 +51,7 @@ int http_c__context_init(struct mosquitto *context)
 	const char *path;
 
 	context->transport = mosq_t_http;
-	context->http_request = mosquitto__calloc(1, 4096); // FIXME - 4096 should be an option
+	context->http_request = mosquitto__calloc(1, (size_t)context->wsd.http_header_size);
 	if(context->http_request == NULL){
 		return MOSQ_ERR_NOMEM;
 	}
@@ -122,7 +122,7 @@ int http_c__read(struct mosquitto *mosq)
 	}
 
 	hlen = strlen(mosq->http_request);
-	read_length = net__read(mosq, &mosq->http_request[hlen], 4096-hlen);
+	read_length = net__read(mosq, &mosq->http_request[hlen], (size_t)mosq->wsd.http_header_size-hlen);
 	if(read_length <= 0){
 		if(read_length == 0){
 			return MOSQ_ERR_CONN_LOST; /* EOF */
diff --git a/lib/mosquitto.c b/lib/mosquitto.c
index fed6e6cd..e24a6916 100644
--- a/lib/mosquitto.c
+++ b/lib/mosquitto.c
@@ -162,6 +162,7 @@ int mosquitto_reinitialise(struct mosquitto *mosq, const char *id, bool clean_st
 	mosq->wsd.mask = UINT8_MAX;
 	mosq->wsd.disconnect_reason = 0xE8;
 	mosq->wsd.is_client = true;
+	mosq->wsd.http_header_size = 4096;
 #endif
 	mosq->transport = mosq_t_tcp;
 	mosq->protocol = mosq_p_mqtt311;
diff --git a/lib/options.c b/lib/options.c
index 53f949cc..bbf190ff 100644
--- a/lib/options.c
+++ b/lib/options.c
@@ -517,6 +517,12 @@ int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t option, int val
 
 		case MOSQ_OPT_HTTP_HEADER_SIZE:
 #if defined(WITH_WEBSOCKETS) && WITH_WEBSOCKETS == LWS_IS_BUILTIN
+			if(value < 100){ /* arbitrary limit */
+				return MOSQ_ERR_INVAL;
+			}else if(mosq->http_request){
+				/* Don't want to resize if part way through the handshake */
+				return MOSQ_ERR_INVAL;
+			}
 			mosq->wsd.http_header_size = value;
 #else
 			return MOSQ_ERR_NOT_SUPPORTED;