--- a/gfx/qcms/transform.c
+++ b/gfx/qcms/transform.c
@@ -1204,17 +1204,21 @@ qcms_transform* qcms_transform_create(
         }
 
 	if (out->output_table_r &&
 			out->output_table_g &&
 			out->output_table_b) {
 		precache = true;
 	}
 
-	if (qcms_supports_iccv4 && (in->A2B0 || out->B2A0 || in->mAB || out->mAB)) {
+	// This precache assumes RGB_SIGNATURE (fails on GRAY_SIGNATURE, for instance)
+	if (qcms_supports_iccv4 &&
+			(in_type == QCMS_DATA_RGB_8 || in_type == QCMS_DATA_RGBA_8) &&
+			(in->A2B0 || out->B2A0 || in->mAB || out->mAB))
+		{
 		// Precache the transformation to a CLUT 33x33x33 in size.
 		// 33 is used by many profiles and works well in pratice. 
 		// This evenly divides 256 into blocks of 8x8x8.
 		// TODO For transforming small data sets of about 200x200 or less
 		// precaching should be avoided.
 		qcms_transform *result = qcms_transform_precacheLUT_float(transform, in, out, 33, in_type);
 		if (!result) {
             		assert(0 && "precacheLUT failed");