diff --git a/EOS/breakout/actual_eos.H b/EOS/breakout/actual_eos.H
index ac0ba24bc..a4b511388 100644
--- a/EOS/breakout/actual_eos.H
+++ b/EOS/breakout/actual_eos.H
@@ -21,6 +21,10 @@ void actual_eos_init ()
         gamma_const = 5.0_rt / 3.0_rt;
     }
 
+    // this EOS assumes that has a 1/mu entry -- make sure that is valid
+    if (aux_names_cxx[AuxZero::iinvmu] != "invmu") {
+        amrex::Error("invalid aux state for breakout EOS");
+    }
 }
 
 template <typename I>
@@ -52,7 +56,7 @@ void actual_eos (I input, T& state)
 
     // Calculate mu. This is the only difference between
     // this EOS and gamma_law.
-    state.mu = 1.0_rt / state.aux[1];
+    state.mu = 1.0_rt / state.aux[AuxZero::iinvmu];
 
     switch (input) {