diff --git a/cram/cram_decode.c b/cram/cram_decode.c
index 39869cbdd..47b7ed076 100644
--- a/cram/cram_decode.c
+++ b/cram/cram_decode.c
@@ -2059,6 +2059,13 @@ static int cram_decode_aux(cram_fd *fd,
                     *has_NM = 1;
             }
         }
+
+        // We could go to 2^32 fine, but we shouldn't be hitting this anyway,
+        // and it's protecting against memory hogs too.
+        if (BLOCK_SIZE(s->aux_blk) > (1u<<31)) {
+            hts_log_error("CRAM->BAM aux block size overflow");
+            goto block_err;
+        }
     }
 
     return r;
diff --git a/cram/cram_encode.c b/cram/cram_encode.c
index 5b56aedd5..9797fa7a8 100644
--- a/cram/cram_encode.c
+++ b/cram/cram_encode.c
@@ -3852,7 +3852,7 @@ int cram_put_bam_seq(cram_fd *fd, bam_seq_t *b) {
 
     if (!c->slice || c->curr_rec == c->max_rec ||
         (bam_ref(b) != c->curr_ref && c->curr_ref >= -1) ||
-        (c->s_num_bases >= fd->bases_per_slice)) {
+        (c->s_num_bases + c->s_aux_bytes >= fd->bases_per_slice)) {
         int slice_rec, curr_rec, multi_seq = fd->multi_seq == 1;
         int curr_ref = c->slice ? c->curr_ref : bam_ref(b);
 
@@ -3885,7 +3885,7 @@ int cram_put_bam_seq(cram_fd *fd, bam_seq_t *b) {
 
         if (CRAM_MAJOR_VERS(fd->version) == 1 ||
             c->curr_rec == c->max_rec || fd->multi_seq != 1 || !c->slice ||
-            c->s_num_bases >= fd->bases_per_slice) {
+            c->s_num_bases + c->s_aux_bytes >= fd->bases_per_slice) {
             if (NULL == (c = cram_next_container(fd, b))) {
                 if (fd->ctr) {
                     // prevent cram_close attempting to flush
@@ -3997,6 +3997,7 @@ int cram_put_bam_seq(cram_fd *fd, bam_seq_t *b) {
     c->curr_rec++;
     c->curr_c_rec++;
     c->s_num_bases += bam_seq_len(b);
+    c->s_aux_bytes += bam_get_l_aux(b);
     c->n_mapped += (bam_flag(b) & BAM_FUNMAP) ? 0 : 1;
     fd->record_counter++;
 
diff --git a/cram/cram_structs.h b/cram/cram_structs.h
index 0a66d51b9..15b7f145b 100644
--- a/cram/cram_structs.h
+++ b/cram/cram_structs.h
@@ -473,6 +473,7 @@ struct cram_container {
     uint32_t crc32;       // CRC32
 
     uint64_t s_num_bases; // number of bases in this slice
+    uint64_t s_aux_bytes; // number of bytes of aux in BAM
 
     uint32_t n_mapped;    // Number of mapped reads
     int ref_free;         // whether 'ref' is owned by us and must be freed.
@@ -500,8 +501,8 @@ typedef struct cram_record {
 
     // Auxiliary data
     int32_t ntags;        // TC
-    int32_t aux;          // idx to s->aux_blk
-    int32_t aux_size;     // total size of packed ntags in aux_blk
+    uint32_t aux;         // idx to s->aux_blk
+    uint32_t aux_size;    // total size of packed ntags in aux_blk
 #ifndef TN_external
     int32_t TN_idx;       // TN; idx to s->TN;
 #else
@@ -509,15 +510,15 @@ typedef struct cram_record {
 #endif
     int     TL;
 
-    int32_t seq;          // idx to s->seqs_blk
-    int32_t qual;         // idx to s->qual_blk
-    int32_t cigar;        // idx to s->cigar
+    uint32_t seq;         // idx to s->seqs_blk
+    uint32_t qual;        // idx to s->qual_blk
+    uint32_t cigar;       // idx to s->cigar
     int32_t ncigar;
     int64_t aend;         // alignment end
     int32_t mqual;        // MQ
 
-    int32_t feature;      // idx to s->feature
-    int32_t nfeature;     // number of features
+    uint32_t feature;     // idx to s->feature
+    uint32_t nfeature;    // number of features
     int32_t mate_flags;   // MF
 } cram_record;
 
@@ -623,8 +624,8 @@ struct cram_slice {
     uint32_t   ncigar;
 
     cram_feature *features;
-    int           nfeatures;
-    int           afeatures; // allocated size of features
+    uint32_t      nfeatures;
+    uint32_t      afeatures; // allocated size of features
 
 #ifndef TN_external
     // TN field (Tag Name)