diff --git a/block/file-posix.c b/block/file-posix.c
index d7fb772c14..a2f6d8a8c8 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2114,7 +2114,12 @@ static int find_allocation(BlockDriverState *bs, off_t start,
     if (offs < 0) {
         return -errno;          /* D3 or D4 */
     }
-    assert(offs >= start);
+
+    if (offs < start) {
+        /* This is not a valid return by lseek().  We are safe to just return
+         * -EIO in this case, and we'll treat it like D4. */
+        return -EIO;
+    }
 
     if (offs > start) {
         /* D2: in hole, next data at offs */
@@ -2146,7 +2151,12 @@ static int find_allocation(BlockDriverState *bs, off_t start,
     if (offs < 0) {
         return -errno;          /* D1 and (H3 or H4) */
     }
-    assert(offs >= start);
+
+    if (offs < start) {
+        /* This is not a valid return by lseek().  We are safe to just return
+         * -EIO in this case, and we'll treat it like H4. */
+        return -EIO;
+    }
 
     if (offs > start) {
         /*