diff --git a/sam.c b/sam.c
index 9f415dbc6..350b8da45 100644
--- a/sam.c
+++ b/sam.c
@@ -6219,14 +6219,24 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
     if (!mm)
         return 0;
     if (mm[0] != 'Z') {
-        hts_log_error("MM tag is not of type Z");
+        hts_log_error("%s: MM tag is not of type Z", bam_get_qname(b));
+        return -1;
+    }
+
+    uint8_t *mi = bam_aux_get(b, "MZ");
+    if (mi && bam_aux2i(mi) != 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.
+        hts_log_error("%s: MM/MZ data length is incompatible with"
+                      " SEQ length", bam_get_qname(b));
         return -1;
     }
 
     uint8_t *ml = bam_aux_get(b, "ML");
     if (!ml) ml = bam_aux_get(b, "Ml");
     if (ml && (ml[0] != 'B' || ml[1] != 'C')) {
-        hts_log_error("ML tag is not of type B,C");
+        hts_log_error("%s: ML tag is not of type B,C", bam_get_qname(b));
         return -1;
     }
     uint8_t *ml_end = ml ? ml+6 + le_to_u32(ml+2) : NULL;
@@ -6312,7 +6322,8 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
 
                     delta = strtol(cp, &cp_end, 10);
                     if (cp_end == cp) {
-                        hts_log_error("Hit end of MM tag. Missing semicolon?");
+                        hts_log_error("%s: Hit end of MM tag. Missing "
+                                      "semicolon?", bam_get_qname(b));
                         return -1;
                     }
 
@@ -6341,8 +6352,8 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
                 state->implicit [mod_num] = implicit;
 
                 if (delta < 0) {
-                    hts_log_error("MM tag refers to bases beyond sequence "
-                                  "length");
+                    hts_log_error("%s: MM tag refers to bases beyond sequence "
+                                  "length", bam_get_qname(b));
                     return -1;
                 }
                 state->MMcount  [mod_num] = delta;
@@ -6357,7 +6368,8 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
                 }
 
                 if (++mod_num >= MAX_BASE_MOD) {
-                    hts_log_error("Too many base modification types");
+                    hts_log_error("%s: Too many base modification types",
+                                  bam_get_qname(b));
                     return -1;
                 }
                 ms++; n++;
@@ -6375,7 +6387,8 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
                     }
                 }
                 if (ml > ml_end) {
-                    hts_log_error("Insufficient number of entries in ML tag");
+                    hts_log_error("%s: Insufficient number of entries in ML "
+                                  "tag", bam_get_qname(b));
                     return -1;
                 }
             } else {
@@ -6387,7 +6400,8 @@ int bam_parse_basemod(const bam1_t *b, hts_base_mod_state *state) {
                         cp++;
             }
             if (!*cp) {
-                hts_log_error("Hit end of MM tag. Missing semicolon?");
+                hts_log_error("%s: Hit end of MM tag. Missing semicolon?",
+                              bam_get_qname(b));
                 return -1;
             }
         }
diff --git a/test/base_mods/MM-MZf1.sam b/test/base_mods/MM-MZf1.sam
new file mode 100644
index 000000000..35074fd05
--- /dev/null
+++ b/test/base_mods/MM-MZf1.sam
@@ -0,0 +1,5 @@
+@SQ	SN:I	LN:999
+r1	0	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240	MZ:i:37
+r1-	16	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:G-m,0,1,4,1,2;G-h,0,7;N-n,17,2;	Ml:B:C,230,204,179,153,128,6,159,240,215
+r2	0	I	4	0	3S33M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240
+r3	0	I	11	0	10S20M6S	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+mh,2,2,0,0,4,1;N+n,15,2;	Ml:B:C,128,0,153,0,0,159,179,0,204,0,230,6,215,240	MZ:i:36
diff --git a/test/base_mods/MM-MZf2.sam b/test/base_mods/MM-MZf2.sam
new file mode 100644
index 000000000..843f93a1b
--- /dev/null
+++ b/test/base_mods/MM-MZf2.sam
@@ -0,0 +1,5 @@
+@SQ	SN:I	LN:999
+r1	0	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240	MZ:i:36
+r1-	16	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:G-m,0,1,4,1,2;G-h,0,7;N-n,17,2;	Ml:B:C,230,204,179,153,128,6,159,240,215
+r2	0	I	4	0	3S33M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240
+r3	0	I	11	0	10S20M6S	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+mh,2,2,0,0,4,1;N+n,15,2;	Ml:B:C,128,0,153,0,0,159,179,0,204,0,230,6,215,240	MZ:f:36
diff --git a/test/base_mods/MM-MZp.sam b/test/base_mods/MM-MZp.sam
new file mode 100644
index 000000000..836a09725
--- /dev/null
+++ b/test/base_mods/MM-MZp.sam
@@ -0,0 +1,5 @@
+@SQ	SN:I	LN:999
+r1	0	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240	MZ:i:36
+r1-	16	I	1	0	36M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:G-m,0,1,4,1,2;G-h,0,7;N-n,17,2;	Ml:B:C,230,204,179,153,128,6,159,240,215
+r2	0	I	4	0	3S33M	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240
+r3	0	I	11	0	10S20M6S	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	DF?GCH88.EG8.7@E9G8A?H9.:C?8,@,,9F@A	Mm:Z:C+mh,2,2,0,0,4,1;N+n,15,2;	Ml:B:C,128,0,153,0,0,159,179,0,204,0,230,6,215,240	MZ:i:36
diff --git a/test/base_mods/MM-multi.sam b/test/base_mods/MM-multi.sam
index b2259a09e..1c7288f50 100644
--- a/test/base_mods/MM-multi.sam
+++ b/test/base_mods/MM-multi.sam
@@ -3,5 +3,5 @@
 @CO	r2 has them combined together, for example as produced by
 @CO	a joint basecaller which assigns probabilities to all
 @CO	trained events simultaneously.
-r1	0	*	0	0	*	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	*	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240
+r1	0	*	0	0	*	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	*	Mm:Z:C+m,2,2,1,4,1;C+h,6,7;N+n,15,2;	Ml:B:C,128,153,179,204,230,159,6,215,240	MZ:i:36
 r2	0	*	0	0	*	*	0	0	AGCTCTCCAGAGTCGNACGCCATYCGCGCGCCACCA	*	Mm:Z:C+mh,2,2,0,0,4,1;N+n,15;	Ml:B:C,77,159,103,133,128,108,154,82,179,57,204,31,240
diff --git a/test/base_mods/base-mods.sh b/test/base_mods/base-mods.sh
index f3f3ca4b7..388ff369e 100755
--- a/test/base_mods/base-mods.sh
+++ b/test/base_mods/base-mods.sh
@@ -31,5 +31,6 @@ test_mod="../test_mod"
 pileup_mod="../pileup_mod"
 
 test_driver $@
+rm _err.tmp _out.tmp
 
 exit $?
diff --git a/test/base_mods/base-mods.tst b/test/base_mods/base-mods.tst
index 3809c0e6e..d246223c8 100644
--- a/test/base_mods/base-mods.tst
+++ b/test/base_mods/base-mods.tst
@@ -42,3 +42,9 @@ P MM-explicit-x.out  $test_mod -x MM-explicit.sam
 # Pileup testing
 P MM-pileup.out $pileup_mod < MM-pileup.sam
 P MM-pileup2.out $pileup_mod < MM-pileup2.sam
+
+# Validation testing.  We just care about exit status here, but the
+# test data is a copy of MM-pileup.sam so that suffices too.
+P MM-pileup.out $pileup_mod < MM-MZp.sam
+F MM-pileup.out $pileup_mod < MM-MZf1.sam
+F MM-pileup.out $pileup_mod < MM-MZf2.sam
diff --git a/test/pileup_mod.c b/test/pileup_mod.c
index 95c353771..323c0c6c2 100644
--- a/test/pileup_mod.c
+++ b/test/pileup_mod.c
@@ -73,7 +73,8 @@ void process_pileup(sam_hdr_t *h, const bam_pileup1_t *p,
 // as each new read is added or removed from the pileups.
 int pileup_cd_create(void *data, const bam1_t *b, bam_pileup_cd *cd) {
     hts_base_mod_state *m = hts_base_mod_state_alloc();
-    bam_parse_basemod(b, m);
+    if (bam_parse_basemod(b, m) < 0)
+        return -1;
     cd->p = m;
     return 0;
 }
@@ -201,7 +202,7 @@ int main(int argc, char **argv) {
     bam_plp_destructor(iter, pileup_cd_destroy);
 
     const bam_pileup1_t *p;
-    int tid, pos, n;
+    int tid, pos, n = 0;
     while ((p = bam_plp_auto(iter, &tid, &pos, &n)) != 0) {
         switch (compact) {
         case 0:
@@ -221,5 +222,5 @@ int main(int argc, char **argv) {
     bam_destroy1(b);
     sam_hdr_destroy(h);
 
-    return 0;
+    return n != 0;
 }