diff --git a/proguard.flags b/proguard.flags
index 1185cca..0fa387a 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -54,3 +54,5 @@
 # Some small BoundedRational methods like equals() are not used by the
 # calculator, but crucial for testing.
 -keepclassmembers class com.android.calculator2.BoundedRational { *; }
+# Need CR comparison operators for testing.
+-keepclassmembers class com.hp.creals.CR { *; }