diff --git a/src/backend_chromium.c b/src/backend_chromium.c
index d4f531a..2171fbf 100644
--- a/src/backend_chromium.c
+++ b/src/backend_chromium.c
@@ -2444,7 +2444,7 @@ bookmark_permute (
         break;
 
       default:
-        unreachable();
+        return -EINVAL;
     }
     ctx->dirty = DIRTY_LEVEL_DATA;
 
diff --git a/src/backend_firefox.c b/src/backend_firefox.c
index f34bf95..2cfe9fd 100644
--- a/src/backend_firefox.c
+++ b/src/backend_firefox.c
@@ -926,6 +926,8 @@ mozbm_pos_shift (
                 return 0;
             }
             *pos_end_ptr = pos_end;
+        } else if (op != BOOKMARKFS_PERMD_OP_MOVE_AFTER) {
+            return -EINVAL;
         }
         ++pos_start;
     } else {
@@ -934,6 +936,8 @@ mozbm_pos_shift (
                 return 0;
             }
             *pos_end_ptr = pos_end;
+        } else if (op != BOOKMARKFS_PERMD_OP_MOVE_BEFORE) {
+            return -EINVAL;
         }
         --pos_start;