diff --git a/libisofs/ecma119.c b/libisofs/ecma119.c
index 106ec62..2a9ee6a 100644
--- a/libisofs/ecma119.c
+++ b/libisofs/ecma119.c
@@ -2013,12 +2013,11 @@ int iso_interval_reader_keep(Ecma119Image *target,
     if (!target->opts->appendable)
         return 0;
 
+    /* --- From here on return either 1 or <0 --- */
+
     /* multi-session write offset must be larger than interval end */
     if (target->opts->ms_block <= ivr->end_byte / BLOCK_SIZE)
-
-        /* >>> ??? return error ??? */
-
-        return 0;
+        return ISO_MULTI_OVER_IMPORTED;
 
     return 1;
 }
@@ -2036,6 +2035,8 @@ int iso_interval_reader_start_size(Ecma119Image *t, char *path,
         return ret;
     *start_byte = ivr->start_byte;
     keep = iso_interval_reader_keep(t, ivr, 0);
+    if (keep < 0)
+        return(keep);
     iso_interval_reader_destroy(&ivr, 0);
     return ISO_SUCCESS + (keep > 0);
 }
@@ -2066,7 +2067,10 @@ int iso_write_partition_file(Ecma119Image *target, char *path,
                                       &ivr, &byte_count, 0);
         if (ret < 0)
             goto ex;
-        if (iso_interval_reader_keep(target, ivr, 0)) {
+        ret = iso_interval_reader_keep(target, ivr, 0);
+        if (ret < 0)
+            goto ex;
+        if (ret > 0) {
             /* From imported_iso and for add-on session. Leave it in place. */
             ret = ISO_SUCCESS;
             goto ex;
diff --git a/libisofs/libisofs.h b/libisofs/libisofs.h
index 8c6977a..090d602 100644
--- a/libisofs/libisofs.h
+++ b/libisofs/libisofs.h
@@ -8887,6 +8887,10 @@ int iso_conv_name_chars(IsoWriteOpts *opts, char *name, size_t name_len,
                                                        (FAILURE, HIGH, -419) */
 #define ISO_SUSP_WRONG_CE_SIZE      0xE830FE5D
 
+/** Multi-session would overwrite imported_iso interval
+                                                       (FAILURE, HIGH, -420) */
+#define ISO_MULTI_OVER_IMPORTED     0xE830FE5C
+
 
 /* Internal developer note: 
    Place new error codes directly above this comment. 
diff --git a/libisofs/messages.c b/libisofs/messages.c
index 84d4cc0..a498a59 100644
--- a/libisofs/messages.c
+++ b/libisofs/messages.c
@@ -551,6 +551,8 @@ const char *iso_error_to_msg(int errcode)
         return "Unable to obtain root directory";
     case ISO_SUSP_WRONG_CE_SIZE:
         return "Zero sized, oversized, or mislocated SUSP CE area found";
+    case ISO_MULTI_OVER_IMPORTED:
+        return "Multi-session would overwrite imported_iso interval";
     default:
         return "Unknown error";
     }
diff --git a/libisofs/system_area.c b/libisofs/system_area.c
index f5a3b6c..1bac1a2 100644
--- a/libisofs/system_area.c
+++ b/libisofs/system_area.c
@@ -129,6 +129,8 @@ static int compute_partition_size(Ecma119Image *t, char *disk_path,
         *size = (byte_count + BLOCK_SIZE - 1) / BLOCK_SIZE;
         keep = iso_interval_reader_keep(t, ivr, 0);
         iso_interval_reader_destroy(&ivr, 0);
+        if (keep < 0)
+            return keep;
         return ISO_SUCCESS + (keep > 0);
     }