diff --git a/sam_mods.c b/sam_mods.c
index fe8db85f77..3263903ce4 100644
--- a/sam_mods.c
+++ b/sam_mods.c
@@ -245,7 +245,7 @@ int bam_parse_basemod2(const bam1_t *b, hts_base_mod_state *state,
     }
 
     uint8_t *mi = bam_aux_get(b, "MN");
-    if (mi && bam_aux2i(mi) != b->core.l_qseq) {
+    if (mi && bam_aux2i(mi) != b->core.l_qseq && b->core.l_qseq) {
         // bam_aux2i with set errno = EINVAL and return 0 if the tag
         // isn't integer, but 0 will be a seq-length mismatch anyway so
         // triggers an error here too.
@@ -359,7 +359,7 @@ int bam_parse_basemod2(const bam1_t *b, hts_base_mod_state *state,
                 if (!cp_end) {
                     // empty list
                     delta = INT_MAX;
-                    cp_end = cp+1;
+                    cp_end = cp;
                 }
             }
             // Now delta is first in list or computed remainder,
@@ -426,6 +426,10 @@ int bam_parse_basemod2(const bam1_t *b, hts_base_mod_state *state,
             }
         }
     }
+    if (ml && ml != ml_end) {
+        hts_log_error("%s: Too many entries in ML tag", bam_get_qname(b));
+        return -1;
+    }
 
     state->nmods = mod_num;
 
@@ -544,8 +548,24 @@ int bam_mods_at_next_pos(const bam1_t *b, hts_base_mod_state *state,
  */
 int bam_next_basemod(const bam1_t *b, hts_base_mod_state *state,
                      hts_base_mod *mods, int n_mods, int *pos) {
-    if (state->seq_pos >= b->core.l_qseq)
+    if (state->seq_pos >= b->core.l_qseq) {
+        // Spots +ve orientation run-overs.
+        // The -ve orientation is spotted in bam_parse_basemod2
+        int i;
+        for (i = 0; i < state->nmods; i++) {
+            // Check if any remaining items in MM after hitting the end
+            // of the sequence.
+            if (!b->core.l_qseq)
+                continue;
+
+            if (state->MMcount[i] < 0x7f000000 ||
+                (*state->MM[i]!=0 && *state->MM[i]!=';')) {
+                hts_log_warning("MM tag refers to bases beyond sequence length");
+                return -1;
+            }
+        }
         return 0;
+    }
 
     // Look through state->MMcount arrays to see when the next lowest is
     // per base type;
@@ -579,18 +599,6 @@ int bam_next_basemod(const bam1_t *b, hts_base_mod_state *state,
     }
     *pos = state->seq_pos = i;
 
-    if (i >= b->core.l_qseq) {
-        // Check for more MM elements than bases present.
-        for (i = 0; i < state->nmods; i++) {
-            if (!(b->core.flag & BAM_FREVERSE) &&
-                state->MMcount[i] < 0x7f000000) {
-                hts_log_warning("MM tag refers to bases beyond sequence length");
-                return -1;
-            }
-        }
-        return 0;
-    }
-
     if (b->core.flag & BAM_FREVERSE) {
         for (i = 0; i < state->nmods; i++)
             state->MMcount[i] -= freq[seqi_rc[state->canonical[i]]];
diff --git a/test/base_mods/MM-chebi.sam b/test/base_mods/MM-chebi.sam
index 0ec8b9ddb0..475a7d5994 100644
--- a/test/base_mods/MM-chebi.sam
+++ b/test/base_mods/MM-chebi.sam
@@ -1,2 +1,2 @@
 @CO	Separate m, h and N modifications
-*	0	*	0	0	*	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	*	Mm:Z:C+m,2,2,1,4,1;C+76792,6,7;N+n,15;	Ml:B:C,102,128,153,179,204,161,33,212,169
+*	0	*	0	0	*	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	*	Mm:Z:C+m,2,2,1,4,1;C+76792,6,7;N+n,15;	Ml:B:C,102,128,153,179,204,161,33,212