diff --git a/projects/qcms/fuzz.cc b/projects/qcms/fuzz.cc
index 76ed7fc66..752dae6ad 100644
--- a/projects/qcms/fuzz.cc
+++ b/projects/qcms/fuzz.cc
@@ -3,23 +3,26 @@
 #include "qcms.h"
 
 static void transform(qcms_profile* src_profile, qcms_profile* dst_profile,
-                      int alpha) {
+                      size_t size) {
   // qcms supports GRAY and RGB profiles as input, and RGB as output.
 
   uint32_t src_color_space = qcms_profile_get_color_space(src_profile);
-  qcms_data_type src_type = alpha & 1 ? QCMS_DATA_RGBA_8 : QCMS_DATA_RGB_8;
+  qcms_data_type src_type = size & 1 ? QCMS_DATA_RGBA_8 : QCMS_DATA_RGB_8;
   if (src_color_space == icSigGrayData)
-    src_type = alpha & 1 ? QCMS_DATA_GRAYA_8 : QCMS_DATA_GRAY_8;
+    src_type = size & 1 ? QCMS_DATA_GRAYA_8 : QCMS_DATA_GRAY_8;
   else if (src_color_space != icSigRgbData)
     return;
 
   uint32_t dst_color_space = qcms_profile_get_color_space(dst_profile);
   if (dst_color_space != icSigRgbData)
     return;
-  qcms_data_type dst_type = alpha & 2 ? QCMS_DATA_RGBA_8 : QCMS_DATA_RGB_8;
+  qcms_data_type dst_type = size & 2 ? QCMS_DATA_RGBA_8 : QCMS_DATA_RGB_8;
 
   qcms_intent intent = qcms_profile_get_rendering_intent(src_profile);
-  qcms_profile_precache_output_transform(dst_profile);
+  // Firefox calls this on the display profile to increase performance.
+  // Skip with low probability to increase coverage.
+  if (size % 0x10)
+    qcms_profile_precache_output_transform(dst_profile);
 
   qcms_transform* transform = qcms_transform_create(
     src_profile, src_type, dst_profile, dst_type, intent);
@@ -61,8 +64,8 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
     return 0;
   }
 
-  transform(profile, srgb_profile, size & 3);
-  transform(srgb_profile, profile, size & 3);
+  transform(profile, srgb_profile, size);
+  transform(srgb_profile, profile, size);
 
   qcms_profile_release(profile);
   qcms_profile_release(srgb_profile);