diff --git a/libvips/mosaicing/im_lrmerge.c b/libvips/mosaicing/im_lrmerge.c
index f36dee40..cdb012cc 100644
--- a/libvips/mosaicing/im_lrmerge.c
+++ b/libvips/mosaicing/im_lrmerge.c
@@ -392,7 +392,7 @@ make_firstlast( MergeInfo *inf, Overlapping *ovlap, Rect *oreg )
 	int ii; \
 	\
 	for( ii = 0; ii < cb; ii++ ) \
-		if( tt[ii] ) \
+		if( tt[i + ii] ) \
 			break; \
 	if( ii == cb )  \
 		(RESULT) = 1; \
diff --git a/libvips/mosaicing/im_tbmerge.c b/libvips/mosaicing/im_tbmerge.c
index 26af4ef9..09afb6ff 100644
--- a/libvips/mosaicing/im_tbmerge.c
+++ b/libvips/mosaicing/im_tbmerge.c
@@ -335,7 +335,7 @@ make_firstlast( MergeInfo *inf, Overlapping *ovlap, Rect *oreg )
 	int ii; \
 	\
 	for( ii = 0; ii < cb; ii++ ) \
-		if( tt[ii] ) \
+		if( tt[i + ii] ) \
 			break; \
 	if( ii == cb )  \
 		(RESULT) = 1; \