diff --git a/toolchain/check/check_unit.cpp b/toolchain/check/check_unit.cpp
index f4a805ff2b99..6f2908f28072 100644
--- a/toolchain/check/check_unit.cpp
+++ b/toolchain/check/check_unit.cpp
@@ -477,7 +477,8 @@ auto CheckUnit::CheckRequiredDefinitions() -> void {
         CARBON_FATAL("TODO: Support interfaces in DiagnoseMissingDefinitions");
       }
       default: {
-        CARBON_FATAL("Unexpected inst in definitions_required: {0}", decl_inst);
+        CARBON_FATAL("Unexpected inst in definitions_required_by_decl: {0}",
+                     decl_inst);
       }
     }
   }