diff --git a/src/datum_protocol.c b/src/datum_protocol.c
index 1d636f56..d3139481 100644
--- a/src/datum_protocol.c
+++ b/src/datum_protocol.c
@@ -1326,15 +1326,22 @@ int datum_protocol_pow(void *arg) {
 	
 	char * const username = (char *)&msg[i];
 	if (((!datum_config.datum_pool_pass_full_users) && (!datum_config.datum_pool_pass_workers)) || pow->username[0] == '\0') {
-		i+=snprintf(username, 385, "%s", datum_config.mining_pool_address);
+		j = snprintf(username, DATUM_PROTOCOL_MAX_USERNAME_LEN + 1, "%s", datum_config.mining_pool_address);
 	} else if (datum_config.datum_pool_pass_full_users && pow->username[0] != '.') {
 		// TODO: Make sure the usernames are addresses, and if not use one of the configured addresses
-		i+=snprintf(username, 385, "%s", pow->username);
-	} else if (datum_config.datum_pool_pass_full_users || datum_config.datum_pool_pass_workers) {
+		j = snprintf(username, DATUM_PROTOCOL_MAX_USERNAME_LEN + 1, "%s", pow->username);
+	} else {
 		// append the miner's username to the configured address as .workername
-		i+=snprintf(username, 385, "%s%s%s", datum_config.mining_pool_address, (pow->username[0] == '.') ? "" : ".", pow->username);
+		j = snprintf(username, DATUM_PROTOCOL_MAX_USERNAME_LEN + 1, "%s%s%s", datum_config.mining_pool_address, (pow->username[0] == '.') ? "" : ".", pow->username);
+	}
+	if (j < 0) {
+		DLOG_ERROR("Unexpected error copying username to POW!");
+		// Still submit it without a username in case it's a block
+		username[0] = '\0';
+		j = 0;
 	}
-	i++;  // already 0 from snprintf
+	if (j > DATUM_PROTOCOL_MAX_USERNAME_LEN) j = DATUM_PROTOCOL_MAX_USERNAME_LEN;
+	i += j + 1;  // including final null byte
 	
 	// reserve 4 bytes for future use
 	memset(&msg[i], 0, 4); i+=4;
diff --git a/src/datum_protocol.h b/src/datum_protocol.h
index 4782ebb1..a8e2f013 100644
--- a/src/datum_protocol.h
+++ b/src/datum_protocol.h
@@ -50,6 +50,7 @@
 
 #define DATUM_PROTOCOL_MAX_CMD_DATA_SIZE 4194304 // 2^22 - protocol limit!
 #define DATUM_PROTOCOL_BUFFER_SIZE (DATUM_PROTOCOL_MAX_CMD_DATA_SIZE*3)
+#define DATUM_PROTOCOL_MAX_USERNAME_LEN 384
 
 #define MAX_DATUM_CLIENT_EVENTS 32