diff --git a/ChangeLog b/ChangeLog
index 745496df..3e577737 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -8,6 +8,7 @@
 - added im_local_imask(), im_local_dmask()
 - added im_mpercent_hist()
 - im_maplut() casts the index image to one of the uint types
+- fixed a couple of /0 problems with scale == 0 masks
 
 16/1/10 started 7.21.2
 - "invalidate" is careful to keep images alive, so invalidate callbacks can do
diff --git a/TODO b/TODO
index 2ef3eeae..a5ab0a52 100644
--- a/TODO
+++ b/TODO
@@ -4,8 +4,6 @@
 - quite a few hist operations have no GUI ... lhisteq, for example? or
   histspec?
 
-- conv with scale == 0 gives /0 error
-
 - added im_local_imask(), im_local_dmask(), needs docs?
 
   is local_imask() in the best place? shouldn't it be in mask.h?
diff --git a/libvips/convolution/im_conv.c b/libvips/convolution/im_conv.c
index 069854ca..5d5ec9e8 100644
--- a/libvips/convolution/im_conv.c
+++ b/libvips/convolution/im_conv.c
@@ -29,7 +29,7 @@
  *	- reworked and simplified, about 10% faster
  *	- slightly better range clipping
  * 27/7/01 JC
- *	- rejects masks with scale == 0
+ *	- reject masks with scale == 0
  * 7/4/04 
  *	- im_conv() now uses im_embed() with edge stretching on the input, not
  *	  the output
@@ -419,6 +419,10 @@ im_conv_raw( IMAGE *in, IMAGE *out, INTMASK *mask )
 		im_check_noncomplex( "im_conv", in ) ||
 		im_check_imask( "im_conv", mask ) ) 
 		return( -1 );
+	if( mask->scale == 0 ) {
+		im_error( "im_conv", "%s", "mask scale must be non-zero" );
+		return( -1 );
+	}
 	if( !(conv = conv_new( in, out, mask )) )
 		return( -1 );
 
diff --git a/libvips/convolution/im_conv_f.c b/libvips/convolution/im_conv_f.c
index edb58a5a..9b34d93c 100644
--- a/libvips/convolution/im_conv_f.c
+++ b/libvips/convolution/im_conv_f.c
@@ -316,6 +316,10 @@ im_conv_f_raw( IMAGE *in, IMAGE *out, DOUBLEMASK *mask )
 		im_check_noncomplex( "im_conv", in ) || 
 		im_check_dmask( "im_conv", mask ) ) 
 		return( -1 );
+	if( mask->scale == 0 ) {
+		im_error( "im_conv_f", "%s", "mask scale must be non-zero" );
+		return( -1 );
+	}
 	if( !(conv = conv_new( in, out, mask )) )
 		return( -1 );
 
diff --git a/libvips/convolution/im_convsep.c b/libvips/convolution/im_convsep.c
index 8adc52b2..da5da3cd 100644
--- a/libvips/convolution/im_convsep.c
+++ b/libvips/convolution/im_convsep.c
@@ -381,6 +381,10 @@ im_convsep_raw( IMAGE *in, IMAGE *out, INTMASK *mask )
 			"%s", _( "expect 1xN or Nx1 input mask" ) );
                 return( -1 );
 	}
+	if( mask->scale == 0 ) {
+		im_error( "im_convsep", "%s", "mask scale must be non-zero" );
+		return( -1 );
+	}
 	if( !(conv = conv_new( in, out, mask )) )
 		return( -1 );
 
diff --git a/libvips/convolution/im_convsep_f.c b/libvips/convolution/im_convsep_f.c
index 0f752f11..01e9ee8e 100644
--- a/libvips/convolution/im_convsep_f.c
+++ b/libvips/convolution/im_convsep_f.c
@@ -285,6 +285,10 @@ im_convsep_f_raw( IMAGE *in, IMAGE *out, DOUBLEMASK *mask )
 			"%s", _( "expect 1xN or Nx1 input mask" ) );
                 return( -1 );
 	}
+	if( mask->scale == 0 ) {
+		im_error( "im_convsep_f", "%s", "mask scale must be non-zero" );
+		return( -1 );
+	}
 	if( !(conv = conv_new( in, out, mask )) )
 		return( -1 );
 
diff --git a/libvips/mask/rw_mask.c b/libvips/mask/rw_mask.c
index 25d02ebc..8ed9e488 100644
--- a/libvips/mask/rw_mask.c
+++ b/libvips/mask/rw_mask.c
@@ -476,6 +476,8 @@ im_scale_dmask( DOUBLEMASK *m, const char *name )
 
 	if( dsum == m->scale )
 		out->scale = isum;
+	else if( dsum == 0.0 )
+		out->scale = 1.0;
 	else
 		out->scale = IM_RINT( m->scale * isum / dsum );
 
@@ -486,7 +488,7 @@ void
 im_norm_dmask( DOUBLEMASK *mask )
 { 	
 	const int n = mask->xsize * mask->ysize;
-	const double scale = 1.0 / mask->scale;
+	const double scale = (mask->scale == 0) ? 0 : (1.0 / mask->scale);
 
 	int i;