TIFF Group4 compression requires an image depth of 1
diff --git a/coders/tiff.c b/coders/tiff.c
index e2dfcee..8a85420 100755
--- a/coders/tiff.c
+++ b/coders/tiff.c
@@ -3194,6 +3194,7 @@
       case Group4Compression:
       {
         (void) SetImageType(image,BilevelType,exception);
+        (void) SetImageDepth(image,1,exception);
         break;
       }
       case JPEGCompression: