diff --git a/src/ioutil.c b/src/ioutil.c
index 3c92c3b..92b8f5f 100644
--- a/src/ioutil.c
+++ b/src/ioutil.c
@@ -27,7 +27,8 @@ 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;
@@ -41,7 +42,7 @@ int build_allocation_map(struct bitset_mapping* allocation_map, int fd)
 		if (ioctl(fd, FS_IOC_FIEMAP, fiemap) < 0) {
 			debug( "Couldn't get fiemap, returning no allocation_map" );
 			free(allocation_map);
-			return NULL;
+			return 0;
 		}
 
 		for (i=0;i<fiemap->fm_mapped_extents;i++) {
@@ -52,10 +53,24 @@ int build_allocation_map(struct bitset_mapping* allocation_map, int fd)
 			);
 			//debug("range from %ld + %ld", 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;
+		}
+		
 	}
 
 	debug("Successfully built allocation map");
-	return allocation_map;
+	return 1;
 }