diff --git a/lib/scanner/blocks.go b/lib/scanner/blocks.go
index 3c43d64ca..ff670f572 100644
--- a/lib/scanner/blocks.go
+++ b/lib/scanner/blocks.go
@@ -31,7 +31,8 @@ func Blocks(r io.Reader, blocksize int, sizehint int64, counter Counter) ([]prot
 
 	if sizehint > 0 {
 		// Allocate contiguous blocks for the BlockInfo structures and their
-		// hashes once and for all.
+		// hashes once and for all, and stick to the specified size.
+		r = io.LimitReader(r, sizehint)
 		numBlocks := int(sizehint / int64(blocksize))
 		blocks = make([]protocol.BlockInfo, 0, numBlocks)
 		hashes = make([]byte, 0, hashLength*numBlocks)