diff --git a/bam_markdup.c b/bam_markdup.c
index 4c157f7b1..88d0f9829 100644
--- a/bam_markdup.c
+++ b/bam_markdup.c
@@ -530,7 +530,7 @@ static inline int get_coordinate_positions(const char *qname, int *xpos, int *yp
 static int optical_duplicate(bam1_t *ori, bam1_t *dup, long max_dist) {
     int ret = 0, seps;
     char *original, *duplicate;
-    int oxpos, oypos, dxpos, dypos;
+    int oxpos = 0, oypos = 0, dxpos = 0, dypos = 0;
 
 
     original  = bam_get_qname(ori);
@@ -620,12 +620,12 @@ static int mark_duplicates(md_param_t *param, khash_t(duplicates) *dup_hash, bam
 
     if (param->opt_dist) { // mark optical duplicates
         if (optical_duplicate(ori, dup, param->opt_dist)) {
-            bam_aux_append(dup, "dt", 'Z', 3, (uint8_t *) "SQ");
+            bam_aux_append(dup, "dt", 'Z', 3, (const uint8_t *)"SQ");
             dup_type = 'O';
             (*optical)++;
         } else {
             // not an optical duplicate
-            bam_aux_append(dup, "dt", 'Z', 3, (uint8_t *) "LB");
+            bam_aux_append(dup, "dt", 'Z', 3, (const uint8_t *)"LB");
         }
     }
 
@@ -1098,10 +1098,10 @@ static int bam_mark_duplicates(md_param_t *param, char *arg_list, char *out_fn,
 
                     if (param->opt_dist) {
                         if (kh_val(dup_hash, k).type) {
-                            bam_aux_append(b, "dt", 'Z', 3, (uint8_t *) "SQ");
+                            bam_aux_append(b, "dt", 'Z', 3, (const uint8_t *)"SQ");
                             np_opt_duplicate++;
                         } else {
-                            bam_aux_append(b, "dt", 'Z', 3, (uint8_t *) "LB");
+                            bam_aux_append(b, "dt", 'Z', 3, (const uint8_t *)"LB");
                         }
                     }
                 }