diff --git a/blkmaker.c b/blkmaker.c
index 7bc1421..68f3272 100644
--- a/blkmaker.c
+++ b/blkmaker.c
@@ -47,6 +47,8 @@ bool _blkmk_dblsha256(void *hash, const void *data, size_t datasz) {
 
 #define dblsha256 _blkmk_dblsha256
 
+#define max_varint_size (9)
+
 uint64_t blkmk_init_generation3(blktemplate_t * const tmpl, const void * const script, const size_t scriptsz, bool * const inout_newcb) {
 	if (tmpl->cbtxn && !(*inout_newcb && (tmpl->mutations & BMM_GENERATE)))
 	{
@@ -482,29 +484,48 @@ char varintEncode(unsigned char *out, uint64_t n) {
 
 char *blkmk_assemble_submission_(blktemplate_t * const tmpl, const unsigned char * const data, const unsigned int dataid, blknonce_t nonce, const bool foreign)
 {
-	unsigned char blk[80 + 8 + 1000000];
+	const bool incl_gentxn = (foreign || (!(tmpl->mutations & BMAb_TRUNCATE && !dataid)));
+	const bool incl_alltxn = (foreign || !(tmpl->mutations & BMAb_COINBASE));
+	
+	size_t blkbuf_sz = libblkmaker_blkheader_size;
+	if (incl_gentxn) {
+		blkbuf_sz += max_varint_size + tmpl->cbtxn->datasz;
+		if (incl_alltxn) {
+			blkbuf_sz += tmpl->txns_datasz;
+		}
+	}
+	
+	unsigned char * const blk = malloc(blkbuf_sz);
+	if (!blk) {
+		return NULL;
+	}
+	
 	memcpy(blk, data, 76);
 	nonce = htonl(nonce);
 	memcpy(&blk[76], &nonce, 4);
 	size_t offs = 80;
 	
-	if (foreign || (!(tmpl->mutations & BMAb_TRUNCATE && !dataid)))
-	{
+	if (incl_gentxn) {
 		offs += varintEncode(&blk[offs], 1 + tmpl->txncount);
 		
 		if (!_blkmk_extranonce(tmpl, &blk[offs], dataid, &offs))
+		{
+			free(blk);
 			return NULL;
+		}
 		
-		if (foreign || !(tmpl->mutations & BMAb_COINBASE))
+		if (incl_alltxn) {
 			for (unsigned long i = 0; i < tmpl->txncount; ++i)
 			{
 				memcpy(&blk[offs], tmpl->txns[i].data, tmpl->txns[i].datasz);
 				offs += tmpl->txns[i].datasz;
 			}
+		}
 	}
 	
 	char *blkhex = malloc((offs * 2) + 1);
 	_blkmk_bin2hex(blkhex, blk, offs);
+	free(blk);
 	
 	return blkhex;
 }
diff --git a/blktemplate.h b/blktemplate.h
index d8aa1e4..90049b0 100644
--- a/blktemplate.h
+++ b/blktemplate.h
@@ -24,6 +24,8 @@ typedef uint32_t blktime_t;
 typedef int16_t blktime_diff_t;
 typedef uint32_t blknonce_t;
 
+#define libblkmaker_blkheader_size (80)
+
 struct blktxn_t {
 	unsigned char *data;
 	size_t datasz;