diff --git a/ChangeLog b/ChangeLog
index 02e88cc6..4fb3d070 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,9 @@
 - add vips_rect_overlapsrect()
 - composite is much faster at positioning subimages
 
+17/12/18 started 8.7.4
+- mapim was not offsetting by window offset [erdmann]
+
 21/11/18 started 8.7.3
 - fix infinite loop for autofit with non-scaleable font 
 
diff --git a/libvips/resample/bicubic.cpp b/libvips/resample/bicubic.cpp
index bdd612f8..f247ec29 100644
--- a/libvips/resample/bicubic.cpp
+++ b/libvips/resample/bicubic.cpp
@@ -533,17 +533,17 @@ vips_interpolate_bicubic_interpolate( VipsInterpolate *interpolate,
 	const int bands = in->im->Bands;
 	const int lskip = VIPS_REGION_LSKIP( in );
 
-	g_assert( ix - 1 >= in->valid.left );
-	g_assert( iy - 1 >= in->valid.top );
-	g_assert( ix + 2 < VIPS_RECT_RIGHT( &in->valid ) );
-	g_assert( iy + 2 < VIPS_RECT_BOTTOM( &in->valid ) );
-
 	/* Confirm that absolute_x and absolute_y are >= 1, because of
 	 * window_offset.
 	 */
 	g_assert( x >= 1.0 );
 	g_assert( y >= 1.0 );
 
+	g_assert( ix - 1 >= in->valid.left );
+	g_assert( iy - 1 >= in->valid.top );
+	g_assert( ix + 2 < VIPS_RECT_RIGHT( &in->valid ) );
+	g_assert( iy + 2 < VIPS_RECT_BOTTOM( &in->valid ) );
+
 #ifdef DEBUG
 	printf( "vips_interpolate_bicubic_interpolate: %g %g\n", x, y );
 	printf( "\tleft=%d, top=%d, width=%d, height=%d\n",
diff --git a/libvips/resample/mapim.c b/libvips/resample/mapim.c
index 9e0b30f7..b82960fa 100644
--- a/libvips/resample/mapim.c
+++ b/libvips/resample/mapim.c
@@ -5,6 +5,8 @@
  * 12/8/18
  * 	- prevent float->int overflow
  * 	- a bit quicker
+ * 17/12/18
+ * 	- we were not offsetting pixel fetches by window_offset
  */
 
 /*
@@ -229,8 +231,8 @@ vips_mapim_region_minmax( VipsRegion *region, VipsRect *r, VipsRect *bounds )
 	TYPE * restrict p1 = (TYPE *) p; \
 	\
 	for( x = 0; x < r->width; x++ ) { \
-		TYPE px = p1[0]; \
-		TYPE py = p1[1]; \
+		TYPE px = p1[0] + window_offset; \
+		TYPE py = p1[1] + window_offset; \
 		\
 		if( px >= clip_width || \
 			py >= clip_height ) { \
@@ -249,8 +251,8 @@ vips_mapim_region_minmax( VipsRegion *region, VipsRect *r, VipsRect *bounds )
 	TYPE * restrict p1 = (TYPE *) p; \
 	\
 	for( x = 0; x < r->width; x++ ) { \
-		TYPE px = p1[0]; \
-		TYPE py = p1[1]; \
+		TYPE px = p1[0] + window_offset; \
+		TYPE py = p1[1] + window_offset; \
 		\
 		if( px < 0 || \
 			px >= clip_width || \
@@ -278,6 +280,8 @@ vips_mapim_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop )
 	const VipsImage *in = in_array[0];
 	const int window_size = 
 		vips_interpolate_get_window_size( mapim->interpolate );
+	const int window_offset = 
+		vips_interpolate_get_window_offset( mapim->interpolate );
 	const VipsInterpolateMethod interpolate = 
 		vips_interpolate_get_method( mapim->interpolate );
 	const int ps = VIPS_IMAGE_SIZEOF_PEL( in );
@@ -310,6 +314,7 @@ vips_mapim_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop )
 
 	/* The bounding box of that area is what we will need from @in. Add
 	 * enough for the interpolation stencil as well.
+	 * 
 	 */
 	bounds.width += window_size - 1;
 	bounds.height += window_size - 1;
diff --git a/test/test-suite/test_resample.py b/test/test-suite/test_resample.py
index 4e4713ae..1375718b 100644
--- a/test/test-suite/test_resample.py
+++ b/test/test-suite/test_resample.py
@@ -198,6 +198,14 @@ class TestResample:
         b = im.crop(50, 0, im.width - 50, im.height).gaussblur(2)
         assert (a - b).abs().max() < 20
 
+        # this was a bug at one point, strangely, if executed with debug
+        # enabled
+        mp = pyvips.Image.xyz(100, 100)
+        interp = pyvips.Interpolate.new('bicubic')
+        assert im.mapim(mp, interpolate=interp).avg() == \
+                   im.crop(0, 0, 100, 100).avg()
+
+
 
 if __name__ == '__main__':
     pytest.main()