diff --git a/fs_mgr/liblp/partition_opener.cpp b/fs_mgr/liblp/partition_opener.cpp
index 3b12213..cc4a882 100644
--- a/fs_mgr/liblp/partition_opener.cpp
+++ b/fs_mgr/liblp/partition_opener.cpp
@@ -64,6 +64,12 @@
         PERROR << __PRETTY_FUNCTION__ << "BLKALIGNOFF failed on " << block_device;
         return false;
     }
+    // The kernel can return -1 here when misaligned devices are stacked (i.e.
+    // device-mapper).
+    if (alignment_offset == -1) {
+        alignment_offset = 0;
+    }
+
     int logical_block_size;
     if (ioctl(fd, BLKSSZGET, &logical_block_size) < 0) {
         PERROR << __PRETTY_FUNCTION__ << "BLKSSZGET failed on " << block_device;