diff --git a/external/parthenon b/external/parthenon
index 374d08c6..f9e41049 160000
--- a/external/parthenon
+++ b/external/parthenon
@@ -1 +1 @@
-Subproject commit 374d08c66d1137951816a017c28a201392d46310
+Subproject commit f9e41049178586c26de0c0b069cf7d05075b019d
diff --git a/kharma/kharma.hpp b/kharma/kharma.hpp
index 323c533e..91c9a99c 100644
--- a/kharma/kharma.hpp
+++ b/kharma/kharma.hpp
@@ -89,19 +89,23 @@ Packages_t ProcessPackages(std::unique_ptr<ParameterInput>& pin);
  * Check whether a given field is anywhere in outputs.
  * Used to avoid calculating expensive fields (jcon, divB) if they
  * will not even be written.
+ * Note this compares the field name as a substring rather than
+ * an exact match to a vector element, so sub-names like `prims.`
+ * or `coords.` will match any field which contains them.
  */
 inline bool FieldIsOutput(ParameterInput *pin, std::string name)
 {
     InputBlock *pib = pin->pfirst_block;
     while (pib != nullptr) {
-        if (pib->block_name.compare(0, 16, "parthenon/output") == 0 &&
+        // For every output block with a 'variables' entry...
+        if (pib->block_name.find("parthenon/output") != std::string::npos &&
             pin->DoesParameterExist(pib->block_name, "variables")) {
             std::string allvars = pin->GetString(pib->block_name, "variables");
             if (allvars.find(name) != std::string::npos) {
                 return true;
             }
         }
-        pib = pib->pnext; // move to next input block name
+        pib = pib->pnext;
     }
     return false;
 }