diff --git a/ChangeLog b/ChangeLog
index e6dae0a4..500cbb77 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -26,6 +26,7 @@
 - better fitting of fonts with overhanging edges [Adrià]
 - revise C++ example [fangqiao]
 - strict round down on jpeg shrink on load [davidwood]
+- configure test for g++ 7.2 and composite.cpp
 
 12/2/18 started 8.6.3
 - use pkg-config to find libjpeg, if we can
diff --git a/configure.ac b/configure.ac
index 057c39d8..67507c02 100644
--- a/configure.ac
+++ b/configure.ac
@@ -416,6 +416,30 @@ if test x$have_vector_shuffle = x"yes"; then
     have_vector_arith=no
   ])
   AC_LANG_POP([C++])
+fi
+
+# gcc 7.2 seems to work, but then gets confused by signed constants in 
+# templates
+if test x$have_vector_arith = x"yes"; then
+  AC_MSG_CHECKING([for C++ signed constants in vector templates])
+  AC_LANG_PUSH([C++])
+  AC_TRY_COMPILE([
+    typedef float v4f __attribute__((vector_size(4 * sizeof(float))));
+    template <typename T>
+    static void
+    h( v4f B )
+    {
+        v4f f;
+        f = -1 * B;
+    }
+  ],[
+  ],[
+    AC_MSG_RESULT([yes])
+  ], [
+    AC_MSG_RESULT([no])
+    have_vector_arith=no
+  ])
+  AC_LANG_POP([C++])
 
   if test x$have_vector_arith = x"yes"; then
     AC_DEFINE_UNQUOTED(HAVE_VECTOR_ARITH, 1,
diff --git a/libvips/colour/icc_transform.c b/libvips/colour/icc_transform.c
index 152c2470..ba8439e8 100644
--- a/libvips/colour/icc_transform.c
+++ b/libvips/colour/icc_transform.c
@@ -546,6 +546,7 @@ vips_image_expected_sig( VipsImage *image )
 		 */
 		switch( image->Bands ) {
 		case 1:
+		case 2:
 			expected_sig = cmsSigGrayData;
 			break;
 
@@ -554,6 +555,7 @@ vips_image_expected_sig( VipsImage *image )
 			break;
 
 		case 4:
+		case 5:
 			expected_sig = cmsSigCmykData;
 			break;