diff --git a/src/add-ons/kernel/file_systems/bfs/BlockAllocator.cpp b/src/add-ons/kernel/file_systems/bfs/BlockAllocator.cpp
index b3dcd55d82..af6a3cdb37 100644
--- a/src/add-ons/kernel/file_systems/bfs/BlockAllocator.cpp
+++ b/src/add-ons/kernel/file_systems/bfs/BlockAllocator.cpp
@@ -1514,7 +1514,7 @@ BlockAllocator::CheckBlocks(off_t start, off_t length, bool allocated)
 		return B_BAD_VALUE;
 
 	uint32 group = start >> fVolume->AllocationGroupShift();
-	uint32 groupBlock = start % (fVolume->BlockSize() << 3);
+	uint32 groupBlock = start / (fVolume->BlockSize() << 3);
 	uint32 blockOffset = start % fVolume->BlockSize();
 
 	AllocationBlock cached(fVolume);
@@ -1530,6 +1530,8 @@ BlockAllocator::CheckBlocks(off_t start, off_t length, bool allocated)
 			}
 		}
 
+		blockOffset = 0;
+
 		if (++groupBlock >= fGroups[group].NumBlocks()) {
 			groupBlock = 0;
 			group++;