diff --git a/src/ioutil.c b/src/ioutil.c
index 93f977d..0c2bf90 100644
--- a/src/ioutil.c
+++ b/src/ioutil.c
@@ -28,7 +28,8 @@ int build_allocation_map(struct bitset_mapping* allocation_map, int fd)
 
 	memset(&fiemap_static, 0, sizeof(fiemap_static));
 
-	for (offset = 0; offset < allocation_map->size; offset += fiemap->fm_length) {
+	for (offset = 0; offset < allocation_map->size; ) {
+		
 		unsigned int i;
 
 		fiemap->fm_start = offset;
@@ -54,6 +55,20 @@ int build_allocation_map(struct bitset_mapping* allocation_map, int fd)
 						  fiemap->fm_extents[i].fe_logical,
 						  fiemap->fm_extents[i].fe_length );
 			}
+		
+		
+			/* must move the offset on, but careful not to jump max_length 
+			 * if we've actually hit max_offsets.
+			 */
+			if (fiemap->fm_mapped_extents > 0) {
+				struct fiemap_extent *last = &fiemap->fm_extents[
+					fiemap->fm_mapped_extents-1
+				];
+				offset += last->fe_logical + last->fe_length;
+			}
+			else {
+				offset += fiemap->fm_length;
+			}
 		}
 	}